tkinter: Delete pack_content from Pack and similarly place_content and grid_content

[Akuli]

At runtime, tkinter does this:

class Misc:
    ...

class Pack:
    ...
    content = pack_content = Misc.pack_content
    slaves = pack_slaves = Misc.pack_slaves

class Place:
    ...
    content = place_content = Misc.place_content
    slaves = place_slaves = Misc.place_slaves

class Grid:
    ...
    content = grid_content = Misc.grid_content
    slaves = place_slaves = Misc.place_slaves

class BaseWidget(Misc):
    ...

class Widget(BaseWidget, Pack, Place, Grid):
    ...

This is very similar to #15763, which already deleted the conflicting content aliases. This PR deletes pack_content, place_content and grid_content from Pack, Place and Grid classes. Anything named foo_content in Python 3.15+ tkinter should be just like foo_slaves, and I have already omitted pack_slaves, grid_slaves and place_slaves from Pack, Grid and Place earlier.

This PR does not delete pack_content, place_content and grid_content entirely. We still have them in class Misc (where the are actually defined), and because all widgets inherit from Misc anyway, these are always available as methods.

While these aliases don't conflict like the content aliases, they aren't useful either. Nobody seems to be doing tkinter.Grid.grid_content(some_widget) when you could just as well do some_widget.grid_content().

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/computation/ops.py:328: error: Need type annotation for "_binary_ops_dict" (hint: "_binary_ops_dict: dict[<type>, <type>] = ...")  [var-annotated]