utime: Because class Clock didn't exist,so remove the declaration in class utime_t.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
Merge pull request #2242 from majianpeng/fix4

utime: Because class Clock didn't exist,so remove the declaration in class utime_t

Reviewed-by: Sage Weil <sage@redhat.com>