remove SingleConstructorOffset.__reduce__

[MarcoGorelli]

__reduce__ isn't documented / part of the public API, but SingleConstructorOffset is used to group together some kinds of offsets, and type checkers can detect some erroneous usage, e.g.

pandas-stubs/tests/test_dtypes.py

Lines 62 to 68 in f5fd35f

	if TYPE_CHECKING_INVALID_USAGE:
	pd.PeriodDtype(
	freq=CustomBusinessDay() # type:ignore[arg-type] # pyright: ignore[reportArgumentType]
	)
	pd.PeriodDtype(
	freq=BusinessDay() # type:ignore[arg-type] # pyright: ignore[reportArgumentType]
	)

• Closes #xxxx (Replace xxxx with the Github issue number)
• Tests added: Please use assert_type() to assert the type of any return value

[Dr-Irv]

thanks @MarcoGorelli