h264_enc: Use free() to release memory allocated with malloc()

mInputFrameData and mSliceGroup are allocated memory with
malloc() but released using delete. Changed this to use free()
instead.

Change-Id: Iecf6684d97fabb2229d9abd94279a24faf9ac2d7
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>