Drop the lightning subpackage
This causes headaches during package updates for the main package
Additionally, it's got its own release schedule independent of the
Thunderbird release schedule and including two such things in a
single SRPM is just a recipe for disaster. Lightning needs to be
packaged up in a separate SRPM.
This is being done in https://bugzilla.redhat.com/show_bug.cgi?id=693493
Conflicts:
thunderbird.spec