posix: Deprecate custom removeAll/mkdirAll implementations. (#4808)

Since go1.8 os.RemoveAll and os.MkdirAll both support long
path names i.e UNC path on windows. The code we are carrying
was directly borrowed from `pkg/os` package and doesn't need
to be in our repo anymore. As a side affect this also
addresses our codecoverage issue.

Refer #4658

cmd/config-migrate.go

@@ -169,7 +169,7 @@ func purgeV1() error {
 		return fmt.Errorf("unrecognized config version ‘%s’", cv1.Version)
 	}
 
-	removeAll(configFile)
+	os.RemoveAll(configFile)
 	log.Println("Removed unsupported config version ‘1’.")
 	return nil
 }