[Docs] Remove custom mock to fix autodoc typing errors

If a libtorrent return type was specified e.g.

   def get_lt_status(self) -> 'lt.torrent_status'

Even as a string autodoc_typehints module would raise and error:

    Handler <function process_docstring at 0x7f6c16c8ec10> for event 'autodoc-process-docstring' threw an exception (exception: getattr(): attribute name must be string)

This was a result of using a custom mock in Sphinx autodoc config and
this Mock object name or qualname returns an object instead of str.

Testing with putting modules in autodoc_mock_imports again showed no
issues so removing custom mock

Ref: tox-dev/sphinx-autodoc-typehints#220

docs/source/conf.py

@@ -219,45 +219,13 @@
 
 # Autodoc section
 # ---------------
-class Mock:
-
-    __all__ = []
-
-    def __init__(self, *args, **kwargs):
-        pass
-
-    def __call__(self, *args, **kwargs):
-        return ''
-
-    @classmethod
-    def __getattr__(cls, name):
-        if name in ('__file__', '__path__', 'xdg_config_home'):
-            return '/dev/null'
-        elif name[0] == name[0].upper():
-            mock_type = type(name, (), {})
-            mock_type.__module__ = __name__
-            return mock_type
-        else:
-            return Mock()
-
-    def __add__(self, other):
-        return other
-
-    def __or__(self, __):
-        return Mock()
-
-
-# Use custom mock as autodoc_mock_imports fails to handle these modules.
-MOCK_MODULES = ['deluge._libtorrent', 'xdg', 'xdg.BaseDirectory']
-
-for mod_name in MOCK_MODULES:
-    sys.modules[mod_name] = Mock()
 
 # Must add these for autodoc to import packages successfully
 builtins.__dict__['_'] = lambda x: x
 builtins.__dict__['_n'] = lambda s, p, n: s if n == 1 else p
 
 autodoc_mock_imports = [
+    'deluge._libtorrent',
     'twisted',
     'rencode',
     'OpenSSL',