Remove colab.py workarounds that broke transformers/hf-hub compatibility

PR #4601 added _pip_install_backend_deps(), _bootstrap_studio_venv(),
and _is_colab() to colab.py as workarounds for install_python_stack
being skipped on Colab. These workarounds:
- Stripped version constraints from studio.txt and installed into system Python
- Upgraded huggingface-hub to >=1.0, breaking Colab's pre-installed
  transformers which requires huggingface-hub<1.0

With install_python_stack now running on Colab (previous commit), these
workarounds are unnecessary — all deps are properly installed by setup.sh.
Restore colab.py to its original PR #4237 structure: just get_colab_url(),
show_link(), and start().

studio/backend/colab.py

@@ -18,90 +18,6 @@ if _backend_dir not in sys.path:
 import _platform_compat  # noqa: F401
 
 
-def _is_colab() -> bool:
-    """Detect Google Colab by checking for COLAB_ prefixed env vars."""
-    import os
-
-    return any(k.startswith("COLAB_") for k in os.environ)
-
-
-def _pip_install_backend_deps() -> None:
-    """Install Studio backend dependencies directly into the current Python.
-
-    Used on Colab when the Studio venv does not exist (install.sh was not
-    run).  Reads the requirements from studio.txt next to this file.
-
-    Version constraints are stripped entirely so pip keeps whatever Colab
-    already has installed (e.g. huggingface-hub, datasets, transformers)
-    and only installs genuinely missing packages like structlog, fastapi.
-    """
-    import re
-    import subprocess
-
-    req_file = Path(__file__).parent / "requirements" / "studio.txt"
-    if not req_file.exists():
-        return
-
-    packages = []
-    for line in req_file.read_text().splitlines():
-        line = line.strip()
-        if not line or line.startswith("#"):
-            continue
-        # Strip all version constraints -- just keep the package name
-        pkg_name = re.split(r"[><=!~;\[]", line)[0].strip()
-        if pkg_name:
-            packages.append(pkg_name)
-
-    if not packages:
-        return
-    print("Installing Studio backend dependencies ...")
-    subprocess.check_call(
-        [sys.executable, "-m", "pip", "install", "-q"] + packages,
-    )
-
-    # Colab ships huggingface-hub 0.36.x which removed is_offline_mode,
-    # breaking transformers. Upgrade to 1.0+ which restored it.
-    try:
-        from huggingface_hub import is_offline_mode  # noqa: F401
-    except ImportError:
-        print("Upgrading huggingface-hub (is_offline_mode missing) ...")
-        subprocess.check_call(
-            [sys.executable, "-m", "pip", "install", "-q", "huggingface-hub>=1.0"],
-        )
-
-
-def _bootstrap_studio_venv() -> None:
-    """Expose the Studio venv's site-packages to the current interpreter.
-
-    On Colab, notebook cells run outside the venv subshell. Instead of
-    installing the full stack into system Python, we prepend the venv's
-    site-packages so that packages like structlog, fastapi, etc. are
-    importable from notebook cells and take priority over system copies.
-
-    If the venv does not exist and we are running on Colab, fall back to
-    pip-installing the backend dependencies into the current environment
-    so that imports like structlog and fastapi succeed.
-    """
-    venv_lib = Path.home() / ".unsloth" / "studio" / "unsloth_studio" / "lib"
-    if not venv_lib.exists():
-        if _is_colab():
-            _pip_install_backend_deps()
-            return
-        import warnings
-
-        warnings.warn(
-            f"Studio venv not found at {venv_lib.parent} -- run 'unsloth studio setup' first",
-            stacklevel = 2,
-        )
-        return
-    for sp in venv_lib.glob("python*/site-packages"):
-        sp_str = str(sp)
-        if sp_str not in sys.path:
-            sys.path.insert(0, sp_str)
-
-
-_bootstrap_studio_venv()
-
 from loggers import get_logger
 
 logger = get_logger(__name__)