[dev.typeparams] runtime, internal/bytealg: remove regabi fallback code on AMD64

[dev.typeparams] runtime, internal/bytealg: remove regabi fallback code on AMD64

As we commit to always enabling register ABI on AMD64, remove the
fallback code.

Change-Id: I30556858ba4bac367495fa94f6a8682ecd771196
Reviewed-on: https://go-review.googlesource.com/c/go/+/341152
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>