5792e76c was too optimistic and did not
invalidate mnCachedMinHeight often enough.
This was resulting in a grey area below table frame when resizing it.

So, revert 5792e76c
"Related bnc#822625: Cache minimum height for table cells."
and istead just use getMinimumWidth() in TableLayouter if that's all we need.
getMinimumHeight() is expensive.

Change-Id: I34c49dda75d6ccccaa5b4d3746114352621a40dd