contrib: remove unused version of os

Caught by test-check-pyflakes.t

Differential Revision: https://phab.mercurial-scm.org/D5471

contrib/fuzz/fm1readmarkers_corpus.py

@@ -1,7 +1,6 b''
 from __future__ import absolute_import, print_function
 
 import argparse
-import os
 import zipfile
 
 ap = argparse.ArgumentParser()