math/big: delete all mulWW assembly code

Now gc can generate the same assembly code.

Change-Id: Iac503003e14045d63e2def66408c13cee516aa37
Reviewed-on: https://go-review.googlesource.com/c/go/+/402575


TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>