Deprecate a couple Channel methods
Deprecate a couple Channel methods

Deprecate `Channel::delete_messages` and `Channel::delete_permission`.

These methods aren't available on all of the variants' types, so they
shouldn't be on the Channel either.

acdenisSK/serenity,Flat/serenity,zeyla/serenity.rs,zeyla/serenity,Roughsketch/serenity,Lakelezz/serenity