Removed prediction_error accumulation

from vp8cx_encode_intra_macro_block.  prediction_error is used when
deciding if a frame should be a keyframe.  After reviewing this with
Yaowu, it was pointed out that vp8cx_encode_intra_macro_block
is only called for keyframes, so the accumulation is unnecessary.

Change-Id: Id79dc81b80d4f5d124f3a0dba1b923887e2e1ec8