fix drop default on postgresql when default value is removed

This is a port of https://github.com/doctrine/dbal/pull/44 which was canceled upstream. It solves the migration issues for me becaus it fixes the wrong SQL. A better fix is hopefully created upstream based aon my analysis https://github.com/doctrine/dbal/pull/44#issuecomment-30434221 doctrine developer @deeky666 will have a look.

@karlitschek @kabum @DeepDiver1975 @PVince81 @bartv2 please review.

PS this PR deprecates: https://github.com/owncloud/core/pull/6346 https://github.com/owncloud/apps/pull/1551 as a more general solution