fix: Remove broken test_get_filter_dirs test

[Holzhaus]

The test depends on the contents of the user's home directory and is therefore considered broken. The test causes issues for end-users, e.g. when they build the python-panflute AUR package on Arch Linux.

The problem can be reproduced using the following steps:

$ file ~/.pandoc ~/.local/share/pandoc
/home/user/.pandoc:             cannot open `/home/user/.pandoc' (No such file or directory)
/home/user/.local/share/pandoc: cannot open `/home/user/.local/share/pandoc' (No such file or directory)

$ pytest -k get_filter_dirs
======================================================== test session starts =========================================================
platform linux -- Python 3.12.3, pytest-8.2.0, pluggy-1.5.0
rootdir: /home/user/Projects/panflute
collected 37 items / 36 deselected / 1 selected
tests/test_panfl.py .                                                                                                          [100%]
================================================== 1 passed, 36 deselected in 0.73s ==================================================

$ mkdir ~/.pandoc

$ pytest -k get_filter_dirs
======================================================== test session starts =========================================================
platform linux -- Python 3.12.3, pytest-8.2.0, pluggy-1.5.0
rootdir: /home/user/Projects/panflute
collected 37 items / 36 deselected / 1 selected
tests/test_panfl.py F                                                                                                          [100%]
============================================================== FAILURES ==============================================================
________________________________________________________ test_get_filter_dirs ________________________________________________________
    def test_get_filter_dirs():
>       assert sorted(pf.get_filter_dirs()) == sorted(pf.get_filter_dirs(hardcoded=False))
E       AssertionError: assert ['/home/user/....ndoc/filters'] == ['/home/user/.pandoc/filters']
E
E         At index 0 diff: '/home/user/.local/share/pandoc/filters' != '/home/user/.pandoc/filters'
E         Use -v to get more diff
tests/test_panfl.py:41: AssertionError
====================================================== short test summary info =======================================================
FAILED tests/test_panfl.py::test_get_filter_dirs - AssertionError: assert ['/home/user/....ndoc/filters'] == ['/home/user/.pandoc/filters']
================================================== 1 failed, 36 deselected in 0.77s ==================================================