In `wp-admin/includes/upgrade.php`, remove dead code:

* In `maybe_create_table()` and `maybe_add_column()`, the response of `$wpdb->query()` is set to a variable that is never used.
* In `make_db_current_silent()`, which is a wrapper for `dbDelta`, a variable is set that is not used. This function appears to be a copy-paste of `make_db_current()`'s `dbDelta()` call but is meant to return nothing, and still does not.  

See #27882.

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