Remove the support for parallel updates

This triggers GitHub's rate limiter...

update_plugins.py

@@ -1,11 +1,3 @@
-try:
-    import concurrent.futures as futures
-except ImportError:
-    try:
-        import futures
-    except ImportError:
-        futures = None
-
 import re
 import shutil
 import tempfile
@@ -106,10 +98,6 @@ if __name__ == "__main__":
     temp_directory = tempfile.mkdtemp()
 
     try:
-        if futures:
-            with futures.ThreadPoolExecutor(16) as executor:
-                executor.map(update, PLUGINS.splitlines())
-        else:
-            [update(x) for x in PLUGINS.splitlines()]
+        [update(x) for x in PLUGINS.splitlines()]
     finally:
         shutil.rmtree(temp_directory)