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/proc_cmdline/proc_cmdline.go

@@ -81,10 +81,6 @@ func (c *procCmdline) FetchUserdata() ([]byte, error) {
 	return cfg, nil
 }
 
-func (c *procCmdline) FetchNetworkConfig(filename string) ([]byte, error) {
-	return nil, nil
-}
-
 func (c *procCmdline) Type() string {
 	return "proc-cmdline"
 }