Now that https://aomedia-review.googlesource.com/#/c/6729/
has been merged, build_intra_predictors_for_interintra() is
now redundant, so replace it by a direct call to
av1_predict_intra_block() and remove the old function.

Reset rect_interintra back to 1.

To do this, we need to make the intra predictor take a
BLOCK_SIZE instead of a TX_SIZE. This is because we need to
be able to predict 32x64 and 64x32 blocks, but there is no
TX_32X64 or TX_64X32.

No effect on output or performance.

Change-Id: I8c185a211c97a85012cc54ec293c785a693608ed