Remove no longer needed threading module import

Last uses of threading were removed in commit
98054da so this commits removes the
imports.

Reviewed By: PrzemekWirkus

Differential Revision: https://reviews.llvm.org/D94665

Author: Thomas Preud'homme

lnt/server/db/v4db.py

@@ -1,11 +1,6 @@
 import glob
 import yaml
 
-try:
-    import threading
-except Exception:
-    import dummy_threading as threading
-
 import sqlalchemy
 
 import lnt.testing