DBTest: remove perf_context's time > 0 check

Summary: DBTest checks  perf_context.seek_internal_seek_time > 0 and perf_context.find_next_user_entry_time > 0, which is not reliable. Remove them.

Test Plan: ./db_test

Reviewers: igor, haobo, ljin

Reviewed By: igor

CC: dhruba, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D16737