To be able to remove SystemGroup, NS_ReleaseOnMainThreadSystemGroup
needs to have its dependency on SystemGroup removed. Since all
releases using SystemGroup would've released on the main thread anyway
we can safely replace NS_ReleaseOnMainThreadSystemGroup with
NS_ReleaseOnMainThread.
Depends on D64390
Differential Revision: https://phabricator.services.mozilla.com/D67631