Remove VP8 save_reg_neon function

This patch did a cleanup following the commit "Save NEON registers
in VP8 NEON functions". The pushing/poping of callee-saved NEON
registers was moved into individual NEON functions. Therefore,
we don't need to save those registers at the beginning of codec.
The related code was removed.

Change-Id: I5648166514fc9beffb780aa138495597731f49ea