configure: drop version checks for in-tree gas [PR91602]

configure: drop version checks for in-tree gas [PR91602]

Special-casing checks for in-tree gas features is unnecessary since
r100007 which made configure-gcc depend on all-gas, and thus making
alternate code path in gcc_GAS_CHECK_FEATURE for in-tree gas
redundant.

Along the way this fixes PR 91602, which is caused by incorrect guess
of leb128 support presence in RISC-V.

First patch removes alternate code path in gcc_GAS_CHECK_FEATURE and
related code, the rest are further cleanups.  Patches 2 and 3 in
series make no functional changes, thus configure is unchanged.

gcc/ChangeLog:

	PR target/91602
	* acinclude.m4 (_gcc_COMPUTE_GAS_VERSION, _gcc_GAS_VERSION_GTE_IFELSE)
	(gcc_GAS_VERSION_GTE_IFELSE): Remove.
	(gcc_GAS_CHECK_FEATURE): Do not handle in-tree case specially.
	* configure.ac: Remove gcc_cv_gas_major_version, gcc_cv_gas_minor_version.
	Remove remaining checks for in-tree assembler.
	* configure: Regenerate.