feat: add remove_string() to storage trait (#8264)

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This PR adds a `remove_string()` API to the Storage trait and also
implements it in the `FileStorage` and `LocalStorage` stucts.

crates/eframe/src/native/file_storage.rs

@@ -159,6 +159,11 @@ impl crate::Storage for FileStorage {
         }
     }
 
+    fn remove_string(&mut self, key: &str) {
+        self.kv.remove(key);
+        self.dirty = true;
+    }
+
     fn flush(&mut self) {
         if self.dirty {
             profiling::scope!("FileStorage::flush");