cmd/internal/obj/x86: remove redundant code (fix TODO)

cmd/internal/obj/x86: remove redundant code (fix TODO)

Special cases described in TODO comments are
fixed in https://golang.org/cl/6901.

One of those blocks was needed until old 6a assembler was removed.
This happened in https://golang.org/cl/12784.

Fixes #24734 (Also contains more details and reasoning)

Change-Id: If1f2f155d36ab236b16ae6f309a0716e00aa6371
Reviewed-on: https://go-review.googlesource.com/105156
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>