Fix: Free global matrices after use.
!4749 (merged) did not free some global matrices/vectors after use causing warnings in like:
warning: There are still 8 global matrices in use. This might be an indicator of a possible waste of memory.
warning: There are still 4 global vectors in use. This might be an indicator of a possible waste of memory.
This MR fixes that. The problem was reported by @PhilippSelzer.