keymap_ui: Fix bug introduced in #35208 (#35237)

Closes #ISSUE

Fixes a bug that was cherry picked onto stable and preview branches
introduced in #35208 whereby modifier keys would show up and not be
removable when editing a keybind

Release Notes:

- (preview only) Keymap Editor: Fixed an issue introduced in v0.197.2
whereby modifier keys would show up and not be removable while recording
keystrokes in the keybind edit modal

crates/settings_ui/src/keybindings.rs

@@ -3114,7 +3114,9 @@ impl KeystrokeInput {
     ) {
         let keystrokes_len = self.keystrokes.len();
 
-        if event.modifiers.is_subset_of(&self.previous_modifiers) {
+        if self.previous_modifiers.modified()
+            && event.modifiers.is_subset_of(&self.previous_modifiers)
+        {
             self.previous_modifiers &= event.modifiers;
             cx.stop_propagation();
             return;