Remove duplicate implementation of intra_build_reference_border.

The search_buildReferenceBorder was an ugly hack and a place for bugs to hide
that should never have existed. Now it doesn't.

The change reduces PSNR a little, but also reduces the bitrate, when the
expected result was to have no change in either. I'm guessing there was still
some bug in the search_buildReferenceBorder, but the bug could also be in
intra_build_reference_border. Will have to do more testing to be sure, but
having one place to look at will be better than having two.