libstdc++: Remove broken std::allocator base classes [PR103340]

The bitmap_allocator, __mt_alloc and __pool_alloc extensions are no
longer suitable for use as the base class of std::allocator, because
they have not been updated to meet the C++20 requirements.  There is a
patch attached to PR 103340 which addresses that, but more work would be
needed to solve the linking errors that occur when the library is
configured to use them.

Using --enable-libstdcxx-allocator=bitmap wouldn't even bootstrap for
the past few years, and I can't find any gcc-testresults reports using
any of these allocators. This patch removes the configure option to use
these as the std::allocator base class. The allocators are still in the
tree and can be used directly, you just can't configure the library to
use one of them as the base class of std::allocator.

libstdc++-v3/ChangeLog:

	PR libstdc++/103340
	PR libstdc++/103400
	PR libstdc++/103381
	* acinclude.m4 (GLIBCXX_ENABLE_ALLOCATOR): Remove mt, bitmap
	and pool options.
	* configure: Regenerate.
	* config/allocator/bitmap_allocator_base.h: Removed.
	* config/allocator/mt_allocator_base.h: Removed.
	* config/allocator/pool_allocator_base.h: Removed.
	* doc/xml/manual/allocator.xml: Update.
	* doc/xml/manual/configure.xml: Update.
	* doc/xml/manual/evolution.xml: Document removal.
	* doc/xml/manual/mt_allocator.xml: Editorial tweaks.
	* doc/html/manual/*: Regenerate.