Back in the old days, shared libraries that linked against libxul/xpcom
were common (binary components), but those days are long gone. Even when
GeckoSharedLibrary was added, most uses were linkage=None. Nowadays, all
of them are, it's time to admit that this should be the default.
Differential Revision: https://phabricator.services.mozilla.com/D238899
13feefedf4