[TASK] Simplify a RelationHandler->readMM() detail

There is a relatively obscure and not well known TCA
config property for type='select': 'allowNonIdValues'.

It can be used in scenarios with 'foreign_table'
relations, where select stores the uids of selected
records as CSV list: It allows additional items
next to "true" relations, for instance items from a
static 'items' list. To keep "true" relations and
"additional static values" apart, those static values
must be some string that can not be interpreted as an
integer uid.

This only works when select stores selections
as CSV list: When using an MM table, relation uids
are forced to be integers. This is documented, too.

RelationHandler still handles 'allowNonIdValues' in
its MM related readMM() method, which is obsolete
code since that can not work.

The patch clean this up and simplifies readMM() a
bit along the way.

Resolves: #103676
Releases: main
Change-Id: I5f760f4c111ce5ae7a1cc3c9412b8900e4ac82d6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83929


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