datasource: remove FetchNetworkConfig step

Its easier to let each datasource grab all metadata in the FetchMetadata
stage than to break it into multiple stages.

datasource/file/file.go

@@ -50,10 +50,6 @@ func (f *localFile) FetchUserdata() ([]byte, error) {
 	return ioutil.ReadFile(f.path)
 }
 
-func (f *localFile) FetchNetworkConfig(filename string) ([]byte, error) {
-	return nil, nil
-}
-
 func (f *localFile) Type() string {
 	return "local-file"
 }