x86: remove always-defined CONFIG_AS_CFI

CONFIG_AS_CFI was introduced by commit e2414910 ("[PATCH] x86:
Detect CFI support in the assembler at runtime"), and extended by
commit f0f12d85 ("x86_64: Check for .cfi_rel_offset in CFI probe").

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_CFI, which is always defined.

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