Only declare the functions in vpx_scale RTCD and include the relevant
header.

Remove unused files and functions in vpx_scale to avoid wasting time
renaming. vpx_scale/win32/scaleopt.c contains functions which have not
been called in a long time but are potentially optimized.

The 'vp8' functions have not been renamed yet. That is for after the
cleanup.

Change-Id: I2c325a101d60fa9d27e7dfcd5b52a864b4a1e09c