Revert two diffs related to DBIter::FindPrevUserKey()

Revert two diffs related to DBIter::FindPrevUserKey()

Summary:
This diff reverts the following two previous diffs related to
DBIter::FindPrevUserKey(), which makes db_stress unstable.
We should bake a better fix for this.

* "Fix a comparison in DBIter::FindPrevUserKey()"
  ec70fea4c4025351190eba7a02bd09bb5f083790.

* "Fixed endless loop in DBIter::FindPrevUserKey()"
  acee2b08a2d37154b8f9e2dc74b1966202c15ec5.

Test Plan: db_stress

Reviewers: anthony, igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D41301
(cherry picked from commit 685582a0b43cb19cb12bf7122657f9ea0f8c52c4)