arm: fix compilation for HIGH_BIT_DEPTH

Caused by conflicting prototypes for neon predict functions added in
babcabe433d883e0e916aec4b6c352849931a2eb to arm/predict.h.
Remove the duplicate prototypes from arm/predict-c.c and change the type
from pixel to uint8_t since there are no plans for high bit depth arm
neon optimizations.