Delete tkinter.{Grid,Pack,Place}.content

[Akuli]

Python 3.15+ added several things named content or foo_content to tkinter. They should be just like things named slaves or foo_slaves. This PR makes them consistent by deleting aliases named content where aliases named slaves don't exist.

I intentionally omitted the slaves aliases earlier. Nobody has complained here about it or other similar aliases. Basically, tkinter.Grid.content(widget) is just like widget.grid_content(), except that nobody uses the tkinter.Grid.content(widget) form.

The main reason for not having slaves and content aliases is that they confuse type checkers in some complex situations. For example, we already silence a bunch of errors in typeshed when doing multiple inheritance, and for whatever reason, someone might want to use similar multiple inheritance outside typeshed to create some kind of custom widget.

Some history:

• When I created tkinter stubs, I initially left slaves aliases commented out.
• In tkinter Pack,Grid,Place: delete commented-out aliases, explain in allowlist #6518, I deleted the already commented-out slaves aliases.
• In Add Python 3.15 tkinter updates #15753, the content aliases were added even though there are no slaves aliases.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉