Remove /python sourcepath

I feel like this mostly is confusing people.
Also don't see much value for it.

To me it looks like a regular workflow is like:
```
/plugin install ~/src/profanity-plugins/my.py
```

The whole thing with sourcepath, install (which also loads without
having that described anywhere), load etc is confusing.
Also each plugin file that is present in
`.local/share/profanity/plugins` will then be auto loaded. Which means
after installation.

src/config/preferences.h

@@ -157,7 +157,6 @@ typedef enum {
     PREF_ROSTER_COLOR_NICK,
     PREF_OCCUPANTS_COLOR_NICK,
     PREF_BOOKMARK_INVITE,
-    PREF_PLUGINS_SOURCEPATH,
     PREF_ROOM_LIST_CACHE,
     PREF_STATUSBAR_SHOW_NAME,
     PREF_STATUSBAR_SHOW_NUMBER,