[!!!][TASK] Remove $TYPO3_CONF_VARS['FE']['pageOverlayFields']

The configuration $GLOBALS['TYPO3_CONF_VARS']['FE']['pageOverlayFields']
is removed from the default configuration as well as from the overlay
handling in PageRepository and RootlineUtility. This setting has been
used to determine overlay fields in the table pages_language_overlay at
a time in the runtime processing when the complete TCA was not fully
available. Since the allowLanguageSynchronization possibility has been
integrated into TYPO3 CMS 8, l10n_mode was available already and the TCA
is loaded as well, the pageOverlayFields settings are superfluous.

Change-Id: I36198d7dec94204fc761048de705aa03a5e931c3
Resolves: #80149
Releases: master
Reviewed-on: https://review.typo3.org/51954


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>