ext: Remove dead code from main.py

This has been tested with vulture:
https://pypi.org/project/vulture

JIRA: https://gem5.atlassian.net/projects/GEM5/issues/GEM5-533

Change-Id: I5f0fbe6c4f5620503c03dfb1b3c8eb1fac31409e
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30243
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

ext/testlib/main.py

@@ -30,7 +30,6 @@ import os
 import itertools
 
 import testlib.configuration as configuration
-import testlib.fixture as fixture_mod
 import testlib.handlers as handlers
 import testlib.loader as loader_mod
 import testlib.log as log