MMR: move RPC code from frame/ to client/ (#12805)

* mmr: move MMR RPC from frame/ to client/

Signed-off-by: Adrian Catangiu <adrian@parity.io>

* client/mmr: adjust logging levels to avoid spam

* cargo fmt

* remove unused imports

Signed-off-by: Adrian Catangiu <adrian@parity.io>

substrate/Cargo.toml

@@ -42,6 +42,7 @@ members = [
 	"client/informant",
 	"client/keystore",
 	"client/merkle-mountain-range",
+	"client/merkle-mountain-range/rpc",
 	"client/network",
 	"client/network-gossip",
 	"client/network/bitswap",
@@ -108,7 +109,6 @@ members = [
 	"frame/lottery",
 	"frame/membership",
 	"frame/merkle-mountain-range",
-	"frame/merkle-mountain-range/rpc",
 	"frame/multisig",
 	"frame/nicks",
 	"frame/node-authorization",