[TASK] Remove empty constructors in EXT:form

An empty constructor just calling parent is not needed, because parent
constructors are always called automatically.

Change-Id: I89384aa584048108fab444e3ee0d50e661c7d014
Fixes: #38335
Releases: 6.0
Reviewed-on: http://review.typo3.org/12314
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn