[CLEANUP] Simplify updating of timestamp in writeForeignField

There is no need to check the record we are updating -
since we update it anyway, update timestamp too and
save both a call to getRecord and a loop through fields,
casting and checking each field value comparing to DB.

Change-Id: Ic807030c5cea83d36e543ed0f75cf980c3872d1e
Releases: master, 7.6
Resolves: #79004
Reviewed-on: https://review.typo3.org/50976


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Tested-by: Philipp Gampe <philipp.gampe@typo3.org>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>