Remove unused kf rate variables

Remove tot_key_frame_bits and prior_key_frame_size[] as they were
tracked but never used. Remove intra_frame_target, as it was only
used to initialize prior_key_frame_size.

Refactor vp8_adjust_key_frame_context() some to remove unnecessary
calculations.

Change-Id: Icbc2c83d2b90e184be03e6f9679e678f3a4bce8f