Revert "osd: detect (some) misordered ondisk tmaps"

This reverts commit 10b68b2e1a1479c478843fab8232537675053c9b.

The check for 'nextkey < last_disk_key' makes not much sense since
last_disk_key is an empty string and not set before. Comparing a
decoded string to be less than an empty string will be never true.

Since this if() isn't part of a loop last_disk_key is only set
once and there is no other consumer: revert this dead code.

Conflicts:
	src/osd/ReplicatedPG.cc
Merge pull request #2897 from ceph/wip-da-revert-10b68b

Revert "osd: detect (some) misordered ondisk tmaps"

Reviewed-by: Sage Weil <sage@redhat.com>
Reviewed-by: Samuel Just <sjust@redhat.com>