oci: appendOSMounts(): remove unused error, and move

This function was added in ae22854e2b, but never
returned an error, and the error-return was not handled on the callsite. This
patch removes the unused error return, and moves it to a file related to mounts,
which allowed for some of the stubs to be removed and shared between non-FreeBSD
platforms.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

oci/spec_opts_windows.go

@@ -115,7 +115,3 @@ func escapeAndCombineArgs(args []string) string {
 	}
 	return strings.Join(escaped, " ")
 }
-
-func appendOSMounts(s *Spec, os string) error {
-	return nil
-}