runtime: minor improvement of string scanning

If we set obj, then it will be enqueued for marking at the end of the scanning loop.
This is not necessary, since we've already marked it.
This can wait for 1.4 if you wish.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/80030043

src/pkg/runtime/mgc0.c

@@ -913,10 +913,8 @@ scanblock(Workbuf *wbuf, bool keepworking)
 
 		case GC_STRING:
 			stringptr = (String*)(stack_top.b + pc[1]);
-			if(stringptr->len != 0) {
-				obj = stringptr->str;
-				markonly(obj);
-			}
+			if(stringptr->len != 0)
+				markonly(stringptr->str);
 			pc += 2;
 			continue;