cmd/gc: remove ignored debugging arguments in Fconv print

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/123880043

src/cmd/gc/mparith1.c

@@ -591,7 +591,7 @@ Fconv(Fmt *fp)
 			d = mpgetflt(fvp);
 			if(d >= 0 && (fp->flags & FmtSign))
 				fmtprint(fp, "+");
-			return fmtprint(fp, "%g", d, exp, fvp);
+			return fmtprint(fp, "%g", d);
 		}
 		
 		// very out of range. compute decimal approximation by hand.