core: auto-purge plugin contributions on plugin unload

Call theme_unregister_plugin (plugin) from plugin_remove, right after
config_file_free_all_plugin and before unhook_all_plugin. This drops
every theme contribution whose plugin pointer matches the plugin being
unloaded, so subsequent /theme apply runs no longer try to set
options for an unloaded plugin and per-plugin contributions don't
outlive the plugin.

Script-owned contributions (plugin != NULL && script != NULL) are
left intact - they are cleaned up per-script in the next commit.

This commit is wiring only: the underlying theme_unregister_plugin
function and its semantics are already covered by
TEST(CoreTheme, UnregisterByOwner) in the previous commit.

src/plugins/plugin.c

@@ -1217,6 +1217,9 @@ plugin_remove (struct t_weechat_plugin *plugin)
     /* remove all configuration files */
     config_file_free_all_plugin (plugin);
 
+    /* remove all theme contributions registered by this plugin */
+    theme_unregister_plugin (plugin);
+
     /* remove all hooks */
     unhook_all_plugin (plugin, NULL);