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_defs.c

@@ -2136,7 +2136,6 @@ static struct cmd_t command_defs[] = {
     { "/plugins",
       parse_args, 0, 3, NULL,
       CMD_SUBFUNCS(
-              { "sourcepath", cmd_plugins_sourcepath },
               { "install", cmd_plugins_install },
               { "uninstall", cmd_plugins_uninstall },
               { "update", cmd_plugins_update },
@@ -2148,8 +2147,6 @@ static struct cmd_t command_defs[] = {
       CMD_NOTAGS
       CMD_SYN(
               "/plugins",
-              "/plugins sourcepath set <path>",
-              "/plugins sourcepath clear",
               "/plugins install [<path>]",
               "/plugins uninstall [<plugin>]",
               "/plugins update [<path>]",
@@ -2160,9 +2157,7 @@ static struct cmd_t command_defs[] = {
       CMD_DESC(
               "Manage plugins. Passing no arguments lists currently loaded plugins.")
       CMD_ARGS(
-              { "sourcepath set <path>", "Set the default path to install plugins from, will be used if no arg is passed to /plugins install." },
-              { "sourcepath clear", "Clear the default plugins source path." },
-              { "install [<path>]", "Install a plugin, or all plugins found in a directory (recursive). Passing no argument will use the sourcepath if one is set." },
+              { "install [<path>]", "Install a plugin, or all plugins found in a directory (recursive)." },
               { "uninstall [<plugin>]", "Uninstall a plugin." },
               { "update [<path>]", "Updates an installed plugin" },
               { "load [<plugin>]", "Load a plugin that already exists in the plugin directory, passing no argument loads all found plugins." },
@@ -2170,7 +2165,6 @@ static struct cmd_t command_defs[] = {
               { "reload [<plugin>]", "Reload a plugin, passing no argument will reload all plugins." },
               { "python_version", "Show the Python interpreter version." })
       CMD_EXAMPLES(
-              "/plugins sourcepath set /home/meee/projects/profanity-plugins",
               "/plugins install",
               "/plugins install /home/steveharris/Downloads/metal.py",
               "/plugins update /home/steveharris/Downloads/metal.py",