Bootstrap/Load: Deprecate `wp_unregister_GLOBALS()`.

The `register_globals` directive in PHP was deprecated in version 5.3 and removed entirely in 5.4.

Now that WordPress only supports PHP 5.6.20 and newer, the `wp_unregister_GLOBALS()` function can be deprecated.

Props ayeshrajans, desrosj, SergeyBiryukov.
Fixes #49938.
Built from https://develop.svn.wordpress.org/trunk@47612


git-svn-id: https://core.svn.wordpress.org/trunk@47387 1a063a9b-81f0-0310-95a4-ce76da25c4cd