[TASK] Remove leftover icon resolving code

Icons within the TCA (iconfile and icons in options) are handled
by the IconRegistry.

The TcaMigration takes care of that.

The code within FormEngine can thus be removed as it is never
executed.

Resolves: #74157
Releases: master
Change-Id: Ia79349193f4a5da0e7046bd63e2221a9e7759d79
Reviewed-on: https://review.typo3.org/47031


Reviewed-by: Daniel Maier <dani-maier@gmx.de>
Tested-by: Daniel Maier <dani-maier@gmx.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frederic Gaus <frederic.gaus@flagbit.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>