Enable clippy::useless_conversion (#8767)

This PR enables the
[`clippy::useless_conversion`](https://rust-lang.github.io/rust-clippy/master/index.html#/useless_conversion)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/collab_ui/src/chat_panel/message_editor.rs

@@ -357,7 +357,7 @@ impl Render for MessageEditor {
             font_size: UiTextSize::Small.rems().into(),
             font_weight: FontWeight::NORMAL,
             font_style: FontStyle::Normal,
-            line_height: relative(1.3).into(),
+            line_height: relative(1.3),
             background_color: None,
             underline: None,
             strikethrough: None,

crates/collab_ui/src/collab_panel.rs

@@ -2171,7 +2171,7 @@ impl CollabPanel {
             font_size: rems(0.875).into(),
             font_weight: FontWeight::NORMAL,
             font_style: FontStyle::Normal,
-            line_height: relative(1.3).into(),
+            line_height: relative(1.3),
             background_color: None,
             underline: None,
             strikethrough: None,