Revert "macOS: Improve deadkeys (#20515)" (#20570)

This reverts commit https://github.com/zed-industries/zed/pull/20515

I'm reverting for now to fix issues with key bindings on Nightly:
* `ctrl-c` and `ctrl-m` are being treated as `ctrl-enter`
* `ctrl-[` isn't working in vim mode
* there's a delay before `cmd-shift-[` switches tabs w/ vim mode enabled

Release Notes:

- N/A

crates/gpui/src/platform/keystroke.rs

@@ -124,9 +124,6 @@ impl Keystroke {
     /// Produces a representation of this key that Parse can understand.
     pub fn unparse(&self) -> String {
         let mut str = String::new();
-        if self.modifiers.function {
-            str.push_str("fn-");
-        }
         if self.modifiers.control {
             str.push_str("ctrl-");
         }