The stub implementation seems like it conflicts on platforms where we have an
implementation, is registered in the child process where we don't need it,
and the implementation itself uses getters where it should use fields.

The code that tries to find a sharepicker implementation handles one not
being registered just fine as far as I can see, so just remove the stub
implementation now that we have actual implementations for people to
look at.

Differential Revision: https://phabricator.services.mozilla.com/D69147

--HG--
extra : moz-landing-system : lando