http_client: Remove unused HttpClient::type_name method (#42803)

Closes #ISSUE

Remove unused method `HttpClient::type_name`. Looking at the PR from a
year ago when it was added, it was never actually used for anything and
seems like a prototyping artifact.

Other misc changes for the `http_client` crate include:

- Use `derive_more::Deref` for `HttpClientWithUrl` (already used for
`HttpClientWithProxy`)
- Move `http_client::proxy()` higher up in the trait definition. (It was
in between methods that have default implementations)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/gpui/src/app.rs

@@ -2400,10 +2400,6 @@ impl HttpClient for NullHttpClient {
     fn proxy(&self) -> Option<&Url> {
         None
     }
-
-    fn type_name(&self) -> &'static str {
-        type_name::<Self>()
-    }
 }
 
 /// A mutable reference to an entity owned by GPUI