REST API: Permit embedding of the 'self' link relation in the /search endpoint.

Removes a special-case prohibition against embedding 'self' which prevented ?_embed from being used with the /wp/v2/search endpoint.

Props TimothyBlynJacobs, chrisvanpatten, kadamwhite.
Fixes #47684.


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git-svn-id: https://core.svn.wordpress.org/trunk@46232 1a063a9b-81f0-0310-95a4-ce76da25c4cd