These files define functions that are unused. Update CMake
and configure builds to remove references and delete the
source files. These files defined the DSPR2 specializations
of high bit depth versions of the following functions:

- av1_iht16x16_256_add
- av1_iht8x8_64_add
- av1_iht4x4_16_add

Change-Id: Ie3ef2592efe1519589a735b0d0db2806eec83e59