Implement MuxedLines::remove_file

[jmagnuson]

Same for MuxedEvents to sync with notify. The mechanics already exist for deregistration between watching and pending; remove_file would just wipe out any entry for a particular PathBuf.

A case could be made that only the absolutified-PathBuf returned from add_file should be accepted (as that is technically the watch 'token'), but accepting either would be more robust, and an error would need to be propagated regardless of pre- or post-absolutification input anyway.