gas/ELF: drop bogus check for ELFOSABI_STANDALONE

obj_elf_parse_section_letters() checking for that ABI, when the checking
at the bottom of obj_elf_section() and the logic in
_bfd_elf_final_write_processing() don't, makes no sense. Either
STANDALONE is meant to be GNU-ish, or it is not, I would think. Drop the
one inconsistent check.

If it was not GNU-ish (as the other two locations suggest), what did the
section23b testcase actually mean to check? The numeric attribute value
0x200000 has an entirely unknown (or more precisely, OS-defined, which
we may or may not know of) meaning then, so ought to be accepted. If it
was GNU-ish, the other testcase, elf/section23a, would want running for
those targets as well, and the testcase in question would still be wrong
to have. Hence that testcase is removed, and section23a is renamed to
just section23.