Unmute does not work for members who have left the guild

### What Red version are you using?

3.4.18

### Cog name

Mutes

### Command name

unmute

### What did you expect to happen?

Expected unmute to remove non-present members

### What actually happened?

unmute fails to remove non-present members from the active mute list

### How can we reproduce this error?

1. mute a member
2. have the member leave the guild
3. attempt to unmute the member
...

### Anything else?

Unmute does not seem to handle any users that have left or been banned in a given guild.
Sorting through the [p]activemutes list has become somewhat tedious, due to the number of members who fit this criteria, as we have no simple means of removing them from the ever growing list....

Start with the Mutes cog's unmute command and reproduce the issue by muting a member, having them leave the guild, and attempting to unmute them. Trace how the command handles members who are no longer present, then verify that unmute removes those users from the active mute list, including users who have left or been banned.

Written by the indexing model from the issue text.