Remove auto-width editor type (#34438)

Closes #34044

`EditorMode::SingleLine { auto_width: true }` was only used for the
title editor in the rules library, and following
https://github.com/zed-industries/zed/pull/31994 we can replace that
with a normal single-line editor without problems. The auto-width editor
was interacting badly with the recently-added newline visualization
code, causing a panic during layout---by switching it to
`Editor::single_line` the newline visualization works there too.

Release Notes:

- Fixed a panic that could occur when opening the rules library.

---------

Co-authored-by: Finn <finn@zed.dev>

crates/editor/src/editor.rs

@@ -482,9 +482,7 @@ pub enum SelectMode {
 
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub enum EditorMode {
-    SingleLine {
-        auto_width: bool,
-    },
+    SingleLine,
     AutoHeight {
         min_lines: usize,
         max_lines: Option<usize>,
@@ -1662,13 +1660,7 @@ impl Editor {
     pub fn single_line(window: &mut Window, cx: &mut Context<Self>) -> Self {
         let buffer = cx.new(|cx| Buffer::local("", cx));
         let buffer = cx.new(|cx| MultiBuffer::singleton(buffer, cx));
-        Self::new(
-            EditorMode::SingleLine { auto_width: false },
-            buffer,
-            None,
-            window,
-            cx,
-        )
+        Self::new(EditorMode::SingleLine, buffer, None, window, cx)
     }
 
     pub fn multi_line(window: &mut Window, cx: &mut Context<Self>) -> Self {
@@ -1677,18 +1669,6 @@ impl Editor {
         Self::new(EditorMode::full(), buffer, None, window, cx)
     }
 
-    pub fn auto_width(window: &mut Window, cx: &mut Context<Self>) -> Self {
-        let buffer = cx.new(|cx| Buffer::local("", cx));
-        let buffer = cx.new(|cx| MultiBuffer::singleton(buffer, cx));
-        Self::new(
-            EditorMode::SingleLine { auto_width: true },
-            buffer,
-            None,
-            window,
-            cx,
-        )
-    }
-
     pub fn auto_height(
         min_lines: usize,
         max_lines: usize,