[TASK] Purge onFocus occurrences from FormEngine

In TYPO3 6.2 palette fields have been handled by an individual on-focus
DOM events, depending on a particular user setting. During the FormEngine
rewrite with TYPO3 CMS 7, these parts have been kept and transported
through the system. Since this information is not used anymore, it can
be removed without any substitution.

Resolves: #75657
Releases: master
Change-Id: I7ab55d3f38b8f34e85027cdbbe8d5850f91dddb5
Reviewed-on: https://review.typo3.org/47699


Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>