This PR reverts a change introduced in PR
https://github.com/emilk/egui/pull/3660 that caused a regression with
`TextEdit::singleline`. The original PR attempted to fix an issue with
the cursor in `TextEdit` inside `ScrollArea`, but it did so by adding
unnecessary size allocation to `TextEdit`, which breaks the layout when
`TextEdit::singleline` is used outside of `ScrollArea`.

The regression introduced by #3660 is more severe, as it completely
breaks the layout of applications using `TextEdit::singleline`, as shown
in the following issues:
* Closes https://github.com/emilk/egui/issues/5500
* Closes https://github.com/emilk/egui/issues/5597
Furthermore, I was unable to reproduce the original bug from PR #3660 in
the current version of egui using the following code:
```rust
impl eframe::App for MyEguiApp {
fn update(&mut self, ctx: &egui::Context, _: &mut eframe::Frame) {
ctx.set_debug_on_hover(true);
egui::CentralPanel::default().show(ctx, |ui| {
ScrollArea::vertical().max_height(100.0).show(ui, |ui| {
ui.add(TextEdit::multiline(&mut self.text).hint_text("Enter text here..."))
});
});
}
}
```
This code attempts to recreate the layout shown in the video from PR
#3660, using a `ScrollArea` with limited height and a `TextEdit` inside.
However, the cursor hiding issue was not reproducible.

Therefore, I believe the code added in PR #3660 is no longer necessary
and only creates more problems.
* Closes https://github.com/emilk/egui/issues/5500
* Closes https://github.com/emilk/egui/issues/5597
* [x] I have followed the instructions in the PR template