We have an SQL mode called PAD_CHAR_TO_FULL_LENGTH that changes CHAR(n)
behavior to be more like ANSI, in that it keeps the full padding when
converting from CHAR to VARCHAR. However, it's poorly supported (ie., indexes
don't seem to heed it), and it doesn't seem to be widely used. Instead of
having it linger in its current half-baked state, just deprecate it for later
removal.

Change-Id: Ib15a74f28797716cbe8f8cc1ae985726bc674ffa