blur: remove horizontal flip
blur: remove horizontal flip

Apparently there was some stale horizontal flip function still there
from my summer of code experiments. Remove it.

ystreet/gst-plugins-gl,ystreet/gst-plugins-gl,mapmapteam/gst-plugins-gl,ystreet/gst-plugins-gl,mapmapteam/gst-plugins-gl,mapmapteam/gst-plugins-gl