DEPR: unused Copy-on-Write option

[rhshadrach]

• closes BUG: SettingWithCopyWarning is not shown with copy-on-write disabled #63230 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md.

[jorisvandenbossche]

Now that CoW is always enabled, the enabling/disabling can simply be removed from the ChainedAssignmentError docstring example:

pandas/pandas/errors/__init__.py

Lines 685 to 691 in fb517ba

	Examples
	--------
	>>> pd.options.mode.copy_on_write = True
	>>> df = pd.DataFrame({"A": [1, 1, 1, 2, 2]}, columns=["A"])
	>>> df["A"][0:3] = 10 # doctest: +SKIP
	... # ChainedAssignmentError: ...
	>>> pd.options.mode.copy_on_write = False

That should fix the doctest failure

[jorisvandenbossche]

Thanks!