Revert "iommu/io-pgtable-arm: Unmap and free table when overwriting with block"

This reverts commit cf27ec93 ("iommu/io-pgtable-arm: Unmap and free
table when overwriting with block").

We have implemented a alternate method for tracking when section tables
should be freed.

Change-Id: I84c256ee3a102e37bb55cb852dd869efa7649f21
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>