Rationalize vp8_rd_pick_intra16x16mby_mode()

Use the function macro_block_yrd() to calculate error and distortion
in keeping with what is done for inter frames.

The old code was using a variance metric for once case and an
SSE function for measuring distortion in the other case.

The function vp8_encode_intra16x16mbyrd() is no longer used.

Change-Id: Ic228cb00a78ff637f4365b43f58fbe5a9273d36f