Enable clippy::useless_conversion (#8767)

This PR enables the
[`clippy::useless_conversion`](https://rust-lang.github.io/rust-clippy/master/index.html#/useless_conversion)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/gpui/src/app/test_context.rs

@@ -335,7 +335,7 @@ impl TestAppContext {
             .map(Keystroke::parse)
             .map(Result::unwrap)
         {
-            self.dispatch_keystroke(window, keystroke.into());
+            self.dispatch_keystroke(window, keystroke);
         }
 
         self.background_executor.run_until_parked()
@@ -347,7 +347,7 @@ impl TestAppContext {
     /// This will also run the background executor until it's parked.
     pub fn simulate_input(&mut self, window: AnyWindowHandle, input: &str) {
         for keystroke in input.split("").map(Keystroke::parse).map(Result::unwrap) {
-            self.dispatch_keystroke(window, keystroke.into());
+            self.dispatch_keystroke(window, keystroke);
         }
 
         self.background_executor.run_until_parked()