Fix: Free global matrices after use.

Christoph Lehmannrequested to merge
chleh/ogs:fix-free-global-matrices into master

!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.

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