x86: remove always-defined CONFIG_AS_AVX

CONFIG_AS_AVX was introduced by commit ea4d26ae ("raid5: add AVX
optimized RAID5 checksumming").

We raise the minimal supported binutils version from time to time.
The last bump was commit 1fb12b35

 ("kbuild: Raise the minimum
required binutils version to 2.21").

I confirmed the code in $(call as-instr,...) can be assembled by the
binutils 2.21 assembler and also by LLVM integrated assembler.

Remove CONFIG_AS_AVX, which is always defined.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Ingo Molnar <mingo@kernel.org>