vp9_highbd_iht8x8_add_neon: rm unused functions
vp9_highbd_iht8x8_add_neon: rm unused functions

their use was removed in:
d8424d289 Fix a bug in vp9_highbd_iht8x8_64_add_neon

Change-Id: I041800f3fb34ffbb7cfa7401370c5a5ceeab01c6

ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx