Embeds: Remove the external oEmbed tests for YouTube.
These tests no longer test anything that WordPress core has control over. YouTube now serves everything
over HTTPS by default, so the tests for #23149 will always pass, and the tests for #32714 aren't testing
anything that core has control over.
Tests for the responses from oEmbed providers has been attempted and reverted in #32360.
Props johnbillion.
Merges [41712] to the 4.6 branch.
See #42076, #32714, #23149.