Remove redundant arguments to CI's flake8 (#6277)

changelog.d/6277.misc

@@ -0,0 +1 @@
+Remove redundant CLI parameters on CI's `flake8` step.

tox.ini

@@ -117,7 +117,7 @@ deps =
     black==19.3b0  # We pin so that our tests don't start failing on new releases of black.
 commands =
     python -m black --check --diff .
-    /bin/sh -c "flake8 synapse tests scripts scripts-dev scripts/hash_password scripts/register_new_matrix_user scripts/synapse_port_db synctl {env:PEP8SUFFIX:}"
+    /bin/sh -c "flake8 synapse tests scripts scripts-dev synctl {env:PEP8SUFFIX:}"
     {toxinidir}/scripts-dev/config-lint.sh
 
 [testenv:check_isort]