cmd/compile: remove regabi magic names
cmd/compile: remove regabi magic names

When developing register ABI, for early testing the compiler
recognized a few magic names to trigger enabling register ABI.
After the development it is disabled (changed to a name that
cannot be spelled in the source code). Later in the development of
register ABI for ARM64 and PPC64, I don't think the magic names
were used. I think they can now be removed.

Keep the magic pragma for now in case it helps development.

Change-Id: Icbc34e2786a80fd8fffe4a464c569dc03a54cd09
Reviewed-on: https://go-review.googlesource.com/c/go/+/393877
Trust: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

golang/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,CAFxX/go,nawawi/go,golang/go,golang/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go