Revert "Optimize Time.utc"

This reverts commit r63848.
It breaks tests.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63849 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

time.c

@@ -1095,9 +1095,6 @@ init_leap_second_info(void)
         timew = timegmw_noleapsecond(&vtm);
 
         number_of_leap_seconds_known = NUM2INT(w2v(wsub(TIMET2WV(known_leap_seconds_limit), rb_time_unmagnify(timew))));
-        if (number_of_leap_seconds_known == 0) {
-            known_leap_seconds_limit = 0;
-        }
     }
 }