Revert "highpass_filter: Refactor the highbd and lowbd versions."

This reverts commit c92c6e9d987defcffc78ee47b8bcfc3e4a233b79.

Reason for revert:

The lowbd version of the function is called even with CONFIG_HIGHBITDEPTH
on, when bit depth == 8. And that particular case will not work as
before with this change.

BUG=aomedia:442

Change-Id: I50c0a1238c93d2c729ab878cf04ca417fee3b202
1 file changed