Cache API: Remove `WP_Object_Cache::__destruct()` and `wpdb::__destruct()`.

Originally added in [4686], these constructor/destructor pairings were designed to prevent the objects from being destroyed before shutdown, when output buffers are flushed.

A deeper investigation reveals that this approach didn't quite work as expected and was later made redundant by introducing `wp_ob_end_flush_all()` in [5462].

Props wonderboymusic, nacin, Mte90, SergeyBiryukov.
Fixes #21402.
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git-svn-id: https://core.svn.wordpress.org/trunk@46907 1a063a9b-81f0-0310-95a4-ce76da25c4cd