Fix regression in TextEdit::singleline layout

[IaVashik]

This PR reverts a change introduced in PR #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 Regression: TextEdit claims full width even with smaller desired_width #5500
• Closes egui egui::TextEdit::singleline long text will make ui.horizontal invalid #5597

Furthermore, I was unable to reproduce the original bug from PR #3660 in the current version of egui using the following code:

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 Regression: TextEdit claims full width even with smaller desired_width #5500
• Closes egui egui::TextEdit::singleline long text will make ui.horizontal invalid #5597
• I have followed the instructions in the PR template

[emilk]

Thanks!

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5640-master
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[emilk]

There are some clippy failures that need fixing