gs-app: Use the libappstream function to check proprietary licenses
This should have equivalent behaviour, but saves us a bit of code and maintenance.
Notably, it handles LicenseRef-proprietary with the same case as our
existing code, so this change doesn’t fix the underlying problem with
issue #1161 (closed). However, the underlying fix is for Steam Link to change its
appdata, and for the appstream spec to be consistent in its casing.
Signed-off-by: Philip Withnall pwithnall@endlessos.org
Fixes: #1161 (closed)