Remove pyfakefs workaround for missing /dev/null

The apb requested GH Actions run for this repo failed on 2020-12-25. Upon
investigation I saw that the error was

FileExistsError: [Errno 17] File exists in the fake filesystem: '/dev/null'

I then checked for recent releases for pyfakefs (and pytest), and saw that
v4.3.3 came out on 2020-12-20. Looking into changes I saw that pyfakefs fixed
the null device name in
pytest-dev/pyfakefs@f1e5163
, which appears to have been the underlying problem that caused us to manually
create '/dev/null' in the first place to workaround pytest needing it for the
caplog fixture.

tests/test_ioc_scan.py

@@ -36,10 +36,6 @@ def test_fs(fs):
     fs.add_real_directory("tests/targets")
     fs.add_real_file("tests/testblob.txt")
 
-    # In the pytest 6.x release something has changed where the caplog fixture
-    # needs to access /dev/null as part of its operation. As a result we need to
-    # manually create it in the fake filesystem.
-    fs.create_file("dev/null")
     yield fs