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/extensions_ui/src/extensions_ui.rs

@@ -368,7 +368,7 @@ impl ExtensionsPage {
             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,