previously remove_redundant_sys () is missing in harfbuzz, after redundant langsys removal, some features are removed as well in prune_features() in fonttools. This change is trying to get the same result between harfbuzz and fonttools.
FontTools removes these when subsetting but harfbuzz does not yet support redundant langsys removal. So this gets the Amiri tests passing for now.
Demonstrates a failure in subsetting GPOS 8/GSUB 6.