paymentsdb: remove kvstore from sql db implementation

Now that every method of the interface was implemented we can
remove the embedded reference we put into place for the sql store
implementation so that the interface would succeed. This is now
removed.

payments/db/sql_store.go

@@ -97,10 +97,6 @@ type BatchedSQLQueries interface {
 
 // SQLStore represents a storage backend.
 type SQLStore struct {
-	// TODO(ziggie): Remove the KVStore once all the interface functions are
-	// implemented.
-	KVStore
-
 	cfg *SQLStoreConfig
 	db  BatchedSQLQueries