Remove 'sys' import from frame_filters.py

pre-commit (really flake8) points out that a recent change to
frame_filters.py left an unused import.  This patch fixes the problem.
diff --git a/gdb/python/lib/gdb/command/frame_filters.py b/gdb/python/lib/gdb/command/frame_filters.py
index 4b2d214..3b7cd03 100644
--- a/gdb/python/lib/gdb/command/frame_filters.py
+++ b/gdb/python/lib/gdb/command/frame_filters.py
@@ -16,8 +16,6 @@
 
 """GDB commands for working with frame-filters."""
 
-import sys
-
 import gdb
 import gdb.frames