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/command/cmd_ac.c

@@ -242,7 +242,6 @@ static Autocomplete console_ac;
 static Autocomplete console_msg_ac;
 static Autocomplete autoping_ac;
 static Autocomplete plugins_ac;
-static Autocomplete plugins_sourcepath_ac;
 static Autocomplete plugins_load_ac;
 static Autocomplete plugins_unload_ac;
 static Autocomplete plugins_reload_ac;
@@ -940,11 +939,6 @@ cmd_ac_init(void)
     autocomplete_add(plugins_ac, "unload");
     autocomplete_add(plugins_ac, "reload");
     autocomplete_add(plugins_ac, "python_version");
-    autocomplete_add(plugins_ac, "sourcepath");
-
-    plugins_sourcepath_ac = autocomplete_new();
-    autocomplete_add(plugins_sourcepath_ac, "set");
-    autocomplete_add(plugins_sourcepath_ac, "clear");
 
     filepath_ac = autocomplete_new();
 
@@ -1355,7 +1349,6 @@ cmd_ac_reset(ProfWin* window)
     autocomplete_reset(console_msg_ac);
     autocomplete_reset(autoping_ac);
     autocomplete_reset(plugins_ac);
-    autocomplete_reset(plugins_sourcepath_ac);
     autocomplete_reset(blocked_ac);
     autocomplete_reset(tray_ac);
     autocomplete_reset(presence_ac);
@@ -2638,21 +2631,10 @@ _plugins_autocomplete(ProfWin* window, const char* const input, gboolean previou
 {
     char* result = NULL;
 
-    if (strncmp(input, "/plugins sourcepath set ", 24) == 0) {
-        return cmd_ac_complete_filepath(input, "/plugins sourcepath set", previous);
-    }
-
     if (strncmp(input, "/plugins install ", 17) == 0) {
         return cmd_ac_complete_filepath(input, "/plugins install", previous);
     }
 
-    if (strncmp(input, "/plugins sourcepath ", 20) == 0) {
-        result = autocomplete_param_with_ac(input, "/plugins sourcepath", plugins_sourcepath_ac, TRUE, previous);
-        if (result) {
-            return result;
-        }
-    }
-
     if (strncmp(input, "/plugins load ", 14) == 0) {
         if (plugins_load_ac == NULL) {
             plugins_load_ac = autocomplete_new();