cmd/compile: remove Value.RemoveArg

It's only used in two places:

 - The one in regalloc.go can be replaced with v.resetArgs()
 - The one in rewrite.go can be open coded

and can cause wrong usage like the bug that CL 358117 fixed.

Change-Id: I125baf237db159d056fe4b1c73072331eea4d06a
Reviewed-on: https://go-review.googlesource.com/c/go/+/357965


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>