MNT: Remove is_path_hidden and walk_skip_hidden

[pllim]

When: Not before v7 (as it was deprecated in v6) but actual timeline might depend on:

• APE 21: APE on ending long term support releases + Updates to APE 2 to reflect new release cycle without LTS astropy-APEs#82

Remove stuff deprecated in #14759 , test_is_path_hidden_deprecation, test_skip_hidden, astropy.utils.tests = data/.hidden_file.txt in package data.

[marcellonascif]

Heyy, I know this issue doesn't need to be done before v7 but I'd like to work on this now and leave a PR so you can use it on v7. If it is possible, I'd like to collaborate to this.
thanks!

[pllim]

No, still a year away. We have not branched for v6.0, let alone v6.1, before v7. Please choose another issue to work on. Thanks for your interest!

[NathanMuniz18]

hello guys, I would like to help with this issue and would like to know if I have permission to contribute to it?

[pllim]

Might have to wait till v8, I think, so not yet? Please find another issue for now. Thanks!