PG: remove weirdness log for last_complete < log.tail

In the case of a divergent object prior to log.tail,
last_complete may end up before log.tail.

Backport: bobtail
Fixes #4174
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit dbadb3e2921297882c5836c67ca32bb8ecdc75db)

Conflicts:

	src/osd/PG.cc