time.c: do not set utc_offset in vtm_add_offset

* time.c (vtm_add_offset): just add offset to other components
  only.  removed subtraction of utc_offset as it is overwritten
  always immediately in callers.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66803 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

time.c

@@ -1911,8 +1911,6 @@ vtm_add_offset(struct vtm *vtm, VALUE off)
     int sec, min, hour;
     int day;
 
-    vtm->utc_offset = subv(vtm->utc_offset, off);
-
     if (lt(off, INT2FIX(0))) {
         sign = -1;
         off = neg(off);