cmd/compile: resolve TODO of Mpflt.SetString

Number literal strings returned by the lexer (internal/syntax package) and other
arguments to SetString never contain leading whitespace. There's no need (anymore)
to trim the argument.

Change-Id: Ib060d109f46f79a364a5c8aa33c4f625fe849264
Reviewed-on: https://go-review.googlesource.com/c/go/+/203997Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -179,11 +179,6 @@ func (a *Mpflt) Neg() {
 }
 
 func (a *Mpflt) SetString(as string) {
-	// TODO(gri) why is this needed?
-	for len(as) > 0 && (as[0] == ' ' || as[0] == '\t') {
-		as = as[1:]
-	}
-
 	f, _, err := a.Val.Parse(as, 0)
 	if err != nil {
 		yyerror("malformed constant: %s (%v)", as, err)