[subset] fix for collect_features and remove_redundant_lamngsys
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.
[subset] Remove redundant langys from Amiri test font.
FontTools removes these when subsetting but harfbuzz does not yet support redundant langsys removal. So this gets the Amiri tests passing for now.