Fix OpenGL directional shadow last split fading

[mrjustaguy]

Fixes #82505

I have no ideas what pssm fade was ment to do, however based on the shader code my guess is that it was an attempt to apply the fading controlled by fade start that got scrapped halfway through the implementation and replaced with a proper implementation, given that I cannot see anything of the sort in the Vulkan Renderer to indicate some other specific feature.

[jsjtxietian]

Can you please squash these commits ? Godot contributing workflow says: “Most of the time, we will prefer a single commit in a given PR (unless there's a good reason to keep the changes separate).”

https://docs.godotengine.org/en/stable/contributing/workflow/pr_review_guidelines.html#get-feedback-from-the-right-people-teams

[mrjustaguy]

Squashed

[clayjohn]

Yep, let's nuke it. This was an accidental copy-paste from the GLES2 code in Godot 3.x

[Calinou]

Tested locally, it works as expected.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉