Remove generation of instance trait by decl_storage. (#6812)

* remove generation of instance trait, no breaking change

* doc

* doc

* Update frame/support/src/traits.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Update frame/support/procedural/src/storage/instance_trait.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/frame/support/src/traits.rs

@@ -1653,6 +1653,17 @@ impl<T> IsType<T> for T {
 	fn into_mut(&mut self) -> &mut T { self }
 }
 
+/// An instance of a pallet in the storage.
+///
+/// It is required that these instances are unique, to support multiple instances per pallet in the same runtime!
+///
+/// E.g. for module MyModule default instance will have prefix "MyModule" and other instances
+/// "InstanceNMyModule".
+pub trait Instance: 'static {
+    /// Unique module prefix. E.g. "InstanceNMyModule" or "MyModule"
+    const PREFIX: &'static str ;
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;