Remove NULL checks from various Channel functions

Passing NULL to these functions are not permitted and must be avoided by the caller
Channel::KickUser() and cmd_kick changes

- Remove uline checks from KickUser()
- Disallow the kick if done by a local user and the victim is ulined in cmd_kick
- Remove call to RemoveAllPrefixes() as it has no additional effect, the Membership object containing the prefixes is destroyed soon after the call
Reduce User* -> Membership* lookups on part and kick
Remove the now unused Channel::RemoveAllPrefixes() and CountInvisible()