Remove contracts RPCs (#12358)

* Remove contracts RPCs

* Remove serde as RPC serialization is no longer needed

* Rename folder to match crate name

* Compile fix

* Remove Byte wrapper

substrate/frame/contracts/runtime-api/README.md

@@ -0,0 +1,7 @@
+Runtime API definition used to provide dry-run capabilities
+
+This API should be imported and implemented by the runtime,
+of a node that wants to provide clients with dry-run
+capabilities.
+
+License: Apache-2.0