remove some assertXxx funcs

keys/storage/filestorage/main.go

@@ -42,13 +42,9 @@ func New(dir string) FileStore {
 	return FileStore{dir}
 }
 
+// assert FileStore satisfies keys.Storage
 var _ keys.Storage = FileStore{}
 
-// assertStorage just makes sure we implement the proper Storage interface
-func (s FileStore) assertStorage() keys.Storage {
-	return s
-}
-
 // Put creates two files, one with the public info as json, the other
 // with the (encoded) private key as gpg ascii-armor style
 func (s FileStore) Put(name string, key []byte, info keys.Info) error {

keys/storage/memstorage/main.go

@@ -22,13 +22,9 @@ func New() MemStore {
 	return MemStore{}
 }
 
+// assert MemStore satisfies keys.Storage
 var _ keys.Storage = MemStore{}
 
-// assertStorage just makes sure we implement the Storage interface
-func (s MemStore) assertStorage() keys.Storage {
-	return s
-}
-
 // Put adds the given key, returns an error if it another key
 // is already stored under this name
 func (s MemStore) Put(name string, key []byte, info keys.Info) error {