Removing kf_{y, uv}_mode_prob arrays from VP9Common.

These arrays have constant values (no any updates). Removing two
corresponding memcpy calls. Making a little cleanup in vp9_entropymode.h
as well: removing redundant 'extern' keyword and moving all function
declarations at the end.

Change-Id: Ia16b38b46aec2e2500f5df29c40a297ae241dede