Removed unnecessary checks in DBTest.ApproximateMemoryUsage
Removed unnecessary checks in DBTest.ApproximateMemoryUsage

Summary:
Just realized that after D45675, part of the code in
DBTest.ApproximateMemoryUsage, does not really test anything anymore, so I
removed it.

Test Plan: make clean all check

Reviewers: rven, igor, sdong, anthony, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D45783

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