Use editor's current font size to scale UI elements (#11844)

This is a follow-up to #11817 and fixes the case where the font size has
been changed with `cmd +/-` and not through the settings.

It now works with both: when the font size is adjusted in the settings
and when changing it via shortcuts.

Release Notes:

- N/A

Demo:


https://github.com/zed-industries/zed/assets/1185253/2e539bd3-f5cc-4aae-9f04-9ae014187959

crates/ui/src/components/icon.rs

@@ -348,14 +348,6 @@ impl Icon {
         self
     }
 
-    pub fn font_size(self, font_size: AbsoluteLength) -> Self {
-        let rems = match font_size {
-            AbsoluteLength::Pixels(pixels) => rems_from_px(pixels.into()),
-            AbsoluteLength::Rems(rems) => rems,
-        };
-        self.custom_size(rems)
-    }
-
     pub fn transform(mut self, transformation: Transformation) -> Self {
         self.transformation = transformation;
         self