[TASK] Get rid of ColumnMap::$relationTablePageIdColumnName

That property always held string "pid" if TCA for the
defined foreign_table exists. This field is only
evaluated at one place in the Backend class, when
persisting new rows into foreign_table. Therefore,
ColumnMap does not need to carry around said value.

Releases: main
Resolves: #100271
Change-Id: I263c1d4e5b6676ceeb46621fdaf2be37169c805c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78229


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Stefan Bürk <stefan@buerk.tech>