chore: Fix some violations of 'needless_pass_by_ref_mut' lint (#18795)

While this lint is allow-by-default, it seems pretty useful to get rid
of mutable borrows when they're not needed.

Closes #ISSUE

Release Notes:

- N/A

crates/theme/src/registry.rs

@@ -189,7 +189,7 @@ impl ThemeRegistry {
     }
 
     /// Removes all themes from the registry.
-    pub fn clear(&mut self) {
+    pub fn clear(&self) {
         self.state.write().themes.clear();
     }
 

crates/theme/src/settings.rs

@@ -498,7 +498,7 @@ pub fn observe_buffer_font_size_adjustment<V: 'static>(
 }
 
 /// Sets the adjusted buffer font size.
-pub fn adjusted_font_size(size: Pixels, cx: &mut AppContext) -> Pixels {
+pub fn adjusted_font_size(size: Pixels, cx: &AppContext) -> Pixels {
     if let Some(AdjustedBufferFontSize(adjusted_size)) = cx.try_global::<AdjustedBufferFontSize>() {
         let buffer_font_size = ThemeSettings::get_global(cx).buffer_font_size;
         let delta = *adjusted_size - buffer_font_size;
@@ -530,7 +530,7 @@ pub fn adjust_buffer_font_size(cx: &mut AppContext, f: fn(&mut Pixels)) {
 }
 
 /// Returns whether the buffer font size has been adjusted.
-pub fn has_adjusted_buffer_font_size(cx: &mut AppContext) -> bool {
+pub fn has_adjusted_buffer_font_size(cx: &AppContext) -> bool {
     cx.has_global::<AdjustedBufferFontSize>()
 }
 
@@ -576,7 +576,7 @@ pub fn adjust_ui_font_size(cx: &mut AppContext, f: fn(&mut Pixels)) {
 }
 
 /// Returns whether the UI font size has been adjusted.
-pub fn has_adjusted_ui_font_size(cx: &mut AppContext) -> bool {
+pub fn has_adjusted_ui_font_size(cx: &AppContext) -> bool {
     cx.has_global::<AdjustedUiFontSize>()
 }