cmd/compile: fix double go prefix trim

CL 106463 introduced what seems to trim the "go:" prefix in pramas
comments twice, so "//go:go:foo" would be handled, too.
So either the strings.TrimPrefix or the [3:]-slicing is not needed.

I opted to remove the [3:]-slicing.

Change-Id: I1325bbc08a9be9ae100c5a7775b0a23f9ed0a419
Reviewed-on: https://go-review.googlesource.com/107256Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -96,7 +96,7 @@ func pragmaValue(verb string) syntax.Pragma {
 func (p *noder) pragcgo(pos syntax.Pos, text string) {
 	f := pragmaFields(text)
 
-	verb := strings.TrimPrefix(f[0][3:], "go:")
+	verb := strings.TrimPrefix(f[0], "go:")
 	f[0] = verb
 
 	switch verb {