This was introduced in last merge with 10.6
The reason is that 10.6 does not need anything special to free histograms
as everything is allocated on a memroot.
In 10.10 histograms is using the vector class, which has some problems:
- No automatic free
- No memory usage accounting
(we should at some point remove vector usage because of the above problem)

Fixed by expliciting freeing histograms when freeing TABLE_STATISTICS
objects.