Dropping a non-master nick nukes channels

Dropping the non-master nick Lepton:
```
15 23:01 [testnet] !services Dropping channel #myon because its access list would 
          be left empty by drop of nickname Myon
15 23:01 [testnet] !services Lepton!~myon@myon.chair dropped nick Lepton
15 23:01 [testnet] !services Lepton!~myon@myon.chair registered nick Lepton
```