Fix a buggy assert

Summary: The assert was pointless since if if prefix is the same as the whole key, assertion will surely fail. Reason behind is when performing the internal key comparison, if user keys are the same, *key with smaller transaction id* wins.

Test Plan: make -j32 && make check

Reviewers: sdong, dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16551