Delete duplicate text when a muted user tries to send a PM

[dax (Daniela)]

When a muted user try to send a personal message a warning will be displayed:

Sorry, is not accepting messages at the moment.
Sorry, %{username} is not accepting messages at the moment.

Only the second sentence should be visible:

[codinghorror (Jeff Atwood)]

I would say this is a bug cc @sam

[nbianca (Bianca)]

This one was a nice bug.

https://github.com/discourse/discourse/pull/6697