Remove `for_stub=True` conditional branch from the `tools.codegenerator.ImportedNamespaces.getvalue`

[junkmd]

THIS IS FOR drop_py2 PLAN! Please see #392.

The ImportedNamespaces is a "sprout class" that generates import parts strings of comtypes.gen._xxxxxxxxxxxx_xxxxxxxx_xxxxxxxx_xxxxxxxxxxxxxxxxxxxxx_x_x_x.

In the past, an optional argument for_stub is added to the getvalues method, since we were trying to add the functionality to generate type stubs at the same time as the executable module under the comtypes.gen(#355).

comtypes/comtypes/tools/codegenerator.py

Line 1402 in 00f2317

def getvalue(self, for_stub=False):

However, as you can see in #400, the plan has been revised and type hints will be added to executable modules with inline annotations as part of the drop_py2 plan.

So it is no longer necessary to take a for_stub argument or conditional branch based on it.

Also, the following doctests are no longer required.

comtypes/comtypes/tools/codegenerator.py

Lines 1332 to 1346 in 00f2317

	>>> print(imports.getvalue(for_stub=True))
	from ctypes import *
	import datetime
	from decimal import Decimal as Decimal
	from comtypes import (
	BSTR as BSTR,
	CoClass as CoClass,
	COMMETHOD as COMMETHOD,
	dispid as dispid,
	DISPMETHOD as DISPMETHOD,
	DISPPROPERTY as DISPPROPERTY,
	GUID as GUID,
	IUnknown as IUnknown,
	)
	import ctypes.wintypes

Before PR, run python -m doctest -v comtypes/tools/codegenerator.py and make sure the test passes.