Summary:
This patch remove the unnecessary Compaction::ReleaseInputs().

Compaction::ReleaseInputs() tries to unref its input_version
and column_family.  However, such unref is always done in
~Compaction(), and all current ReleaseInputs() calls are
right before the destructor.

Test Plan: ./db_test

Reviewers: igor

Reviewed By: igor

Subscribers: igor, rven, dhruba, sdong

Differential Revision: https://reviews.facebook.net/D31605