test_project.py: Remove unused variable in 'with' statement

flake8 reports:

 F841 local variable 'f' is assigned to but never used

Change-Id: If808eb381ee44c7da71e6281615a06a6723cf945
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254593
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>

tests/test_project.py

@@ -163,7 +163,7 @@ class CopyLinkTestCase(unittest.TestCase):
 
   @staticmethod
   def touch(path):
-    with open(path, 'w') as f:
+    with open(path, 'w'):
       pass
 
   def assertExists(self, path, msg=None):