dm: Clean up dm-req-crypt

dm-req-crypt defines optional functions from device mapper. These
functions are not required for dm-req-crypt. dm-req-crypt has few
memory de-allocation issues. This change removes unnecessary
functions and fixes memory deallocation issues as well.

Change-Id: Id36bd13989940ce41571a8a10277730048df6f6d
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>