common/Timer: kill RWTimer

This doesn't work the way we hoped it would, and as of
8253ead1748fc429bf48f5334ee4460ee865d941.  Remove it.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2796 from ceph/wip-rwtimer

common/Timer: kill RWTimer

Reviewed-by: Yehuda Sadeh <yehuda@redhat.com>