Deprecate Block::isValid method

Deprecate Block::isValid method

Block::isValid checks whether the block target equals null. It seems to
have been unused since commit 40a926ab2c566052eefd08c36f246d6b5321afc5.
It was called from Block::newFromDB, which was removed in favour of
Block::newFromTarget, which has its own logic for checking the target.

Since the method is public, it should be deprecated before removal.

Change-Id: Ibcfb377a382f55e80933836960be7e3589908390