Remove unused TKINTER_PROTECT_LOADTK code

The `TKINTER_PROTECT_LOADTK` macro and relevant code (originally for #49372) are only defined for Tk 8.4.13 and earlier, but Tkinter already requires at least 8.5.12. Is there a reason to keep this code around, or can it be removed?


### Linked PRs
* gh-103535
* gh-103542
* gh-103544