PGLog: remove obsolete assert in merge_log

This assert assumes that if olog.head != log.head, olog contains
a log entry at log.head, which may not be true since pg splitting
might have left the log with arbitrary holes.

Related: 0c2769d3321bff6e85ec57c85a08ee0b8e751bcb
Signed-off-by: Samuel Just <sam.just@inktank.com>