Not possible to omit generating those messages, although you could say
$ ansible-lint […] 2>/dev/null
If the positional argument were, say, a group_vars file and it was being linted as if it were a playbook, then "WARNING …" would clearly make sense. Or if you didn't intend "my_test.yml" to be linted as a playbook, then you'd be glad for the warning.
As it is, since you can't pick which lintable schema to use on a given file (at least not from --help's output, or the missing man page), I'll stand by my original statement: it's an informational message that happens to begin with the text "WARNING". That's just my opinion, but ansible-lint is extremely opinionated after all.