[BUGFIX] Remove unnecessary BackendUtility::getProcessedValue() call in GridColumn

Using BackendUtility::getProcessedValue() in GridColumn to
fetch a backend layout column's title is unnecessary and also
leads to an undefined array key warning. The call is therefore
removed. The information is always fetched using the resolved
BackendLayout object. Cases like unassigned columns are handled
at other places already.

Resolves: #102634
Releases: main, 12.4
Change-Id: Icba87de054dd391bb3c89c0e53985e0ec876ddf6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82119


Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>