Simplify Debian repo key installation

The `mopidy.gpg` file is now dearmored, removing the need for `gpg` to
be installed on the enduser's system.

The use of `tee` has been replaced by running `wget` as root. We already
run `wget` as root in the next step, so this isn't something we're
trying to avoid. The main benefit is to avoid using `tee`, which outputs
the binary contents of `mopidy.gpg` to the terminal too, risking that
many users will think something is broken.

docs/installation/debian.rst

@@ -43,8 +43,8 @@ Install from apt.mopidy.com
 #. Add the archive's GPG key::
 
        sudo mkdir -p /usr/local/share/keyrings
-       wget -q -O - https://apt.mopidy.com/mopidy.gpg | gpg --dearmor | \
-         sudo tee /usr/local/share/keyrings/mopidy-archive-keyring.gpg
+       sudo wget -q -O /usr/local/share/keyrings/mopidy-archive-keyring.gpg \
+         https://apt.mopidy.com/mopidy.gpg
 
 #. Add the APT repo to your package sources::