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_linux.go

@@ -176,7 +176,3 @@ func WithRdt(closID, l3CacheSchema, memBwSchema string) SpecOpts {
 func escapeAndCombineArgs(args []string) string {
 	panic("not supported")
 }
-
-func appendOSMounts(s *Spec, os string) error {
-	return nil
-}