[TASK] Clean up RelationHandler variable assignments

This change cleans up some RelationHandler code:
- Working with true boolean inside RelationHandler for getValueArray()
- RelationHandler->MM_oppositeFieldConf is an array, but declared wrongly
as string (since forever!)
- detecting the MM_oppositeTable property should not need a $tmp variable,
  but using a direct assignment - and unset() is not needed due to PHP7s memory management
- internal $isSet flags should be boolean, instead of a number

Resolves: #90066
Releases: master
Change-Id: Ifff2a016a7be945ba6a1dd265e06b1e52bee5de7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62830


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>