Refactor all_font_names (#15345)

In the current code implementation, it seems that the only difference
between `all_font_names` and `all_font_families` is whether dynamically
loaded font resources are included. Specifically, `all_font_families`
returns the names of all system fonts, while `all_font_names` includes
both the system font names and the dynamically loaded font names. In
other words, `all_font_families` is a strict subset of `all_font_names`.
This is what I observed in my tests on macOS.

<img width="682" alt="截屏2024-07-28 00 49 29"
src="https://github.com/user-attachments/assets/47317c28-0074-49d2-bcfa-052cab13e335">

Related codes:
```rust
let x: HashSet<_> = self.all_font_names().into_iter().collect();
let y: HashSet<_> = self.all_font_families().into_iter().collect();
let only_in_x = x.difference(&y).collect::<Vec<_>>();
let only_in_y = y.difference(&x).collect::<Vec<_>>();
println!("=====================================");
println!("1 -> {:?}", only_in_x);
println!("-------------------------------------");
println!("2 -> {:?}", only_in_y);
```

Release Notes:

- N/A

crates/gpui/src/platform/linux/text_system.rs

@@ -77,17 +77,6 @@ impl PlatformTextSystem for CosmicTextSystem {
         result
     }
 
-    fn all_font_families(&self) -> Vec<String> {
-        self.0
-            .read()
-            .font_system
-            .db()
-            .faces()
-            // todo(linux) this will list the same font family multiple times
-            .filter_map(|face| face.families.first().map(|family| family.0.clone()))
-            .collect_vec()
-    }
-
     fn font_id(&self, font: &Font) -> Result<FontId> {
         // todo(linux): Do we need to use CosmicText's Font APIs? Can we consolidate this to use font_kit?
         let mut state = self.0.write();