Turn off some deprecated features

This has ben properly deprecated. Let's turn the codepath off for now,
and see in a few release if we can actually remove

See #353

lib/python/pyflyby/_importdb.py

@@ -28,7 +28,7 @@
 else:
     from typing import Self
 
-SUPPORT_DEPRECATED_BEHAVIOR = True
+SUPPORT_DEPRECATED_BEHAVIOR = False
 
 @memoize
 def _find_etc_dirs():