[BUGFIX] Turn isOnSymmetricSide into static function

Function isOnSymmetricSide of RelationHandler is used as
static function all the time so we properly turn the function
declaration into a static one.

Resolves: #60516
Releases: 6.3, 6.2
Change-Id: I72a922596e69dca26e173dad5b42da76e0949865
Reviewed-on: http://review.typo3.org/31748
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>