x86/kconfig: Remove CONFIG_AS_AVX512

Commit

  5f5305de ("raid6: skip avx512 checks")

and commit

  bc23fe6d ("crypto: x86 - Remove CONFIG_AS_AVX512 handling")

removed all uses of CONFIG_AS_AVX512.

Remove check for assembler support of AVX-512 instructions.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250819085855.333380-4-ubizjak@gmail.com