Remove some unnecessary constructors

Summary:
This is continuing the work done by https://github.com/facebook/rocksdb/commit/27b22f13a300c00c72de7fc826fdae21a734eb49

It's just cleaning up some unnecessary constructors. The most important change is removing Block::Block(const BlockContents& contents) constructor. It was only used from the unit test.

Test Plan: compiles

Reviewers: sdong, yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23547