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.rs

@@ -412,7 +412,6 @@ pub trait PlatformDispatcher: Send + Sync {
 pub(crate) trait PlatformTextSystem: Send + Sync {
     fn add_fonts(&self, fonts: Vec<Cow<'static, [u8]>>) -> Result<()>;
     fn all_font_names(&self) -> Vec<String>;
-    fn all_font_families(&self) -> Vec<String>;
     fn font_id(&self, descriptor: &Font) -> Result<FontId>;
     fn font_metrics(&self, font_id: FontId) -> FontMetrics;
     fn typographic_bounds(&self, font_id: FontId, glyph_id: GlyphId) -> Result<Bounds<f32>>;