REST API: Improve performance by avoiding `call_user_func()`.

The `get_compact_response_links()` method was introduced in WP 4.5, and this conditional is no longer necessary.

Merges [43834] from the 5.0 branch to trunk.

Props schlessera, timothyblynjacobs.
Fixes #45189.

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git-svn-id: https://core.svn.wordpress.org/trunk@44003 1a063a9b-81f0-0310-95a4-ce76da25c4cd