Do not mock json when building the documentation

Building the documentation on Debian unstable with Python 3.7 fails:

```
debian-unstable$ HTML_THEME=saltstack make -C doc html
make: Entering directory 'doc'
No need to update translations. Skipping...
sphinx-build -b html -d _build/doctrees    . _build/html
Running Sphinx v1.7.9
loading translations [en]... done

Exception occurred:
  File "/usr/lib/python3/dist-packages/sphinx/util/jsonimpl.py", line 22, in <module>
    class SphinxJSONEncoder(json.JSONEncoder):
TypeError: __mro_entries__ must return a tuple
The full traceback has been saved in /tmp/sphinx-err-wzl9_n0k.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
make: *** [Makefile:72: html] Error 2
make: Leaving directory 'doc'
debian-unstable$ cat /tmp/sphinx-err-wzl9_n0k.log

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/sphinx/cmdline.py", line 303, in main
    args.warningiserror, args.tags, args.verbosity, args.jobs)
  File "/usr/lib/python3/dist-packages/sphinx/application.py", line 187, in __init__
    self.setup_extension(extension)
  File "/usr/lib/python3/dist-packages/sphinx/application.py", line 411, in setup_extension
    self.registry.load_extension(self, extname)
  File "/usr/lib/python3/dist-packages/sphinx/registry.py", line 315, in load_extension
    mod = __import__(extname, None, None, ['setup'])
  File "/usr/lib/python3/dist-packages/sphinx/builders/applehelp.py", line 20, in <module>
    from sphinx.builders.html import StandaloneHTMLBuilder
  File "/usr/lib/python3/dist-packages/sphinx/builders/html.py", line 43, in <module>
    from sphinx.util import jsonimpl, logging, status_iterator
  File "/usr/lib/python3/dist-packages/sphinx/util/jsonimpl.py", line 22, in <module>
    class SphinxJSONEncoder(json.JSONEncoder):
TypeError: __mro_entries__ must return a tuple
```

The json module is a standard module. I is always present. So do not mock it.

Signed-off-by: Benjamin Drung <benjamin.drung@cloud.ionos.com>

doc/conf.py

@@ -153,7 +153,6 @@ MOCK_MODULES = [
     'napalm',
     'dson',
     'jnpr',
-    'json',
     'lxml',
     'lxml.etree',
     'jnpr.junos',