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/mac/text_system.rs

@@ -5,7 +5,7 @@ use crate::{
 };
 use anyhow::anyhow;
 use cocoa::appkit::{CGFloat, CGPoint};
-use collections::{BTreeSet, HashMap};
+use collections::HashMap;
 use core_foundation::{
     attributed_string::CFMutableAttributedString,
     base::{CFRange, TCFType},
@@ -93,26 +93,18 @@ impl PlatformTextSystem for MacTextSystem {
     }
 
     fn all_font_names(&self) -> Vec<String> {
+        let mut names = Vec::new();
         let collection = core_text::font_collection::create_for_all_families();
         let Some(descriptors) = collection.get_descriptors() else {
-            return Vec::new();
+            return names;
         };
-        let mut names = BTreeSet::new();
         for descriptor in descriptors.into_iter() {
             names.extend(lenient_font_attributes::family_name(&descriptor));
         }
         if let Ok(fonts_in_memory) = self.0.read().memory_source.all_families() {
             names.extend(fonts_in_memory);
         }
-        names.into_iter().collect()
-    }
-
-    fn all_font_families(&self) -> Vec<String> {
-        self.0
-            .read()
-            .system_source
-            .all_families()
-            .expect("core text should never return an error")
+        names
     }
 
     fn font_id(&self, font: &Font) -> Result<FontId> {