docs: Remove section on missing Debian-packaged extensions

For apt.mopidy.com, none of this is an issue.

For Ubuntu:
- none of this is an issue in 15.10 or 16.04 LTS.
- 15.04 is soon out of support.
- 14.04 LTS is old enough that you'll have to read older docs to expect
  facts matching your installation.

For Debian:
- none of this is an issue in sid or stretch.
- Jessie this might be an issue, but the same sentiment as for Ubuntu
  14.04 LTS applies.

Fixes #1399

docs/debian.rst

@@ -112,18 +112,3 @@ from a regular Mopidy setup you'll want to know about.
 - You can check if Mopidy is currently running as a system service by running::
 
       sudo service mopidy status
-
-- Mopidy installed from a Debian package can use Mopidy extensions installed
-  both from Debian packages and with pip. This has always been the case.
-
-  Mopidy installed with pip can use extensions installed with pip, but
-  not extensions installed from a Debian package released before August 2015.
-  This is because the Debian packages used to install extensions into
-  :file:`/usr/share/mopidy` which is normally not on your ``PYTHONPATH``.
-  Thus, your pip-installed Mopidy would not find the Debian package-installed
-  extensions.
-
-  In August 2015, all Mopidy extension Debian packages was modified to install
-  into :file:`/usr/lib/python2.7/dist-packages`, like any other Python Debian
-  package. Thus, Mopidy installed with pip can now use extensions installed
-  from Debian.

docs/installation/debian.rst

@@ -87,44 +87,3 @@ about any other requirements needed for the extension to work properly.
 
 For a full list of available Mopidy extensions, including those not
 installable from apt.mopidy.com, see :ref:`ext`.
-
-
-Missing extensions
-==================
-
-If you've installed a Mopidy extension with pip, restarted Mopidy, and Mopidy
-doesn't find the extension, there's probably a simple explanation and solution.
-
-Mopidy installed with APT can detect and use Mopidy extensions installed with
-both APT and pip. APT installs Mopidy as :file:`/usr/bin/mopidy`.
-
-Mopidy installed with pip can only detect Mopidy extensions installed with pip.
-pip usually installs Mopidy as :file:`/usr/local/bin/mopidy`.
-
-If you have Mopidy installed from both APT and pip, then the pip-installed
-Mopidy will probably shadow the APT-installed Mopidy because
-:file:`/usr/local/bin` usually has precedence over :file:`/usr/bin` in the
-``PATH`` environment variable. To check if this is the case on your system, you
-can use ``which`` to see what installation of Mopidy you use when you run
-``mopidy`` in your shell::
-
-    $ which mopidy
-    /usr/local/bin/mopidy
-
-If this is the case on your system, the recommended solution is to check that
-you have Mopidy installed from APT too::
-
-    $ /usr/bin/mopidy --version
-    Mopidy 0.19.5
-
-And then uninstall the pip-installed Mopidy::
-
-    sudo pip uninstall mopidy
-
-Depending on what shell you use, the shell may still try to use
-:file:`/usr/local/bin/mopidy` even if it no longer exists. Check again with
-``which mopidy`` what your shell believes is the right ``mopidy`` executable to
-run. If the shell is still confused, you may need to restart it, or in the case
-of zsh, run ``rehash`` to update the shell.
-
-For more details on why this works this way, see :ref:`debian`.