Network::clearCaps only sync when caps removed

Add check for if _caps and _capsEnabled contain any capabilities.  If
not, return from the clearCaps call without issuing a network SYNC.

Fixes needless clearCaps calls when reconnecting to a network that's
failing to connect.

This may cause inconsistency if clients miss some sync calls.. but
Quassel protocol currently depends on all sync calls being propogated
anyways.