Fix warnings when building xed 2.0.1 (#264)

* Remove unused variables

Fixes the following warnings when building xed:

../plugins/filebrowser/xed-file-browser-plugin.c: In function ‘xed_file_browser_plugin_activate’:
../plugins/filebrowser/xed-file-browser-plugin.c:532:22: warning: unused variable ‘schema’ [-Wunused-variable]
     GSettingsSchema *schema;
                      ^~~~~~
../plugins/filebrowser/xed-file-browser-plugin.c:531:28: warning: unused variable ‘schema_source’ [-Wunused-variable]
     GSettingsSchemaSource *schema_source;
                            ^~~~~~~~~~~~~

* Remove an unused variable

Fixes the following warning when building xed:

../plugins/sort/xed-sort-plugin.c: In function ‘buffer_sort_lines’:
../plugins/sort/xed-sort-plugin.c:168:12: warning: variable ‘last_line’ set but not used [-Wunused-but-set-variable]
     gchar *last_line = NULL;
            ^~~~~~~~~

* Remove an unused variable

Fixes the following warning when building xed:

../plugins/wordcompletion/xed-wordcompletion-plugin.c: In function ‘xed_wordcompletion_view_deactivate’:
../plugins/wordcompletion/xed-wordcompletion-plugin.c:306:34: warning: unused variable ‘provider’ [-Wunused-variable]
     GtkSourceCompletionProvider *provider;
                                  ^~~~~~~~

* Remove an unused variable

Fixes the following warning when building xed:

../plugins/spell/xed-spell-plugin.c: In function ‘update_ui’:
../plugins/spell/xed-spell-plugin.c:495:24: warning: unused variable ‘buffer’ [-Wunused-variable]
         GtkTextBuffer *buffer;
                        ^~~~~~

plugins/wordcompletion/xed-wordcompletion-plugin.c

@@ -303,7 +303,6 @@ xed_wordcompletion_view_deactivate (XedViewActivatable *activatable)
 {
     XedWordCompletionPluginPrivate *priv;
     GtkSourceCompletion *completion;
-    GtkSourceCompletionProvider *provider;
     GtkTextBuffer *buf;
 
     xed_debug (DEBUG_PLUGINS);