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.c

@@ -1928,8 +1928,6 @@ _get_group(preference_t pref)
     case PREF_BOOKMARK_INVITE:
     case PREF_ROOM_LIST_CACHE:
         return PREF_GROUP_MUC;
-    case PREF_PLUGINS_SOURCEPATH:
-        return PREF_GROUP_PLUGINS;
     case PREF_OMEMO_LOG:
     case PREF_OMEMO_POLICY:
     case PREF_OMEMO_TRUST_MODE:
@@ -2163,8 +2161,6 @@ _get_key(preference_t pref)
         return "color.occupants.nick";
     case PREF_BOOKMARK_INVITE:
         return "bookmark.invite";
-    case PREF_PLUGINS_SOURCEPATH:
-        return "sourcepath";
     case PREF_ROOM_LIST_CACHE:
         return "rooms.cache";
     case PREF_STATUSBAR_SHOW_NAME:

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,