cleanup: Remove flow_tests.py

Flow support is deprecated; testwiki is already locked and mediawiki is
in phase 2a of the deprecation to become read-only shortly.

Bug: T381551
Change-Id: I99986b99904bf0ce0d36ca61341a06fb08abc8bd

Author: xqt

tests/__init__.py

@@ -1,6 +1,6 @@
 """Package tests."""
 #
-# (C) Pywikibot team, 2007-2024
+# (C) Pywikibot team, 2007-2025
 #
 # Distributed under the terms of the MIT license.
 #
@@ -86,7 +86,6 @@ def create_path_func(base_func, subpath):
     'family',
     'file',
     'fixes',
-    'flow',
     'gui',
     'http',
     'i18n',