Removing inv_txm4x4_1_add and inv_txm4x4_add function pointers.

We already have itxm_add member in MACROBLOCKD structure. Both
inv_txm4x4_1_add and inv_txm4x4_add are just its special cases for
different eob values. But eob logic is already implemented in
vp9_iwht4x4_add and vp9_idct4x4_add (that's why also removing
inverse_transform_b_4x4_add).

Change-Id: I80bec9b6f7d40c5e5033c613faca5c819c3e6326