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/epi.rs

@@ -942,6 +942,9 @@ pub trait Storage {
     /// Set the value for the given key.
     fn set_string(&mut self, key: &str, value: String);
 
+    /// Remove a given key.
+    fn remove_string(&mut self, key: &str);
+
     /// write-to-disk or similar
     fn flush(&mut self);
 }