[.flake8]: Remove Deprecated Option --include-in-doctest

The options `--include-in-doctest` and `--exclude-from-doctest` that
allowed the user to specify the files that should be checked by PyFlakes
for doctest syntax
(https://flake8.pycqa.org/en/6.1.0/user/options.html#cmdoption-flake8-include-in-doctest)
have been removed in version 7.0.0 of flake8
(https://flake8.pycqa.org/en/latest/release-notes/7.0.0.html#backwards-incompatible-changes).

Author: andthum

.flake8

@@ -80,8 +80,6 @@ extend-select =
     W504,
 # List of glob patterns to include for checks.
 filename = *.py
-# Files that are checked by PyFlakes for doctest syntax.
-include-in-doctest = *.py, *.rst
 # Number of spaces used for indentation.
 indent-size = 4
 # Maximum allowed McCabe complexity