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

* In `get_core_checksums()`, `$return` is set and never used.
* In `core_update_footer()`, `break` is unnecessary and unreachable after `return`.
* In `get_theme_updates()`, `$themes` is set and never used.
* In `wp_theme_update_row()`, `$theme_name` is the only user of `$themes_allowedtags`... neither are used. 	

See #27882.


git-svn-id: https://develop.svn.wordpress.org/trunk@28304 602fd350-edb4-49c9-b593-d223f7449a82
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.


git-svn-id: https://develop.svn.wordpress.org/trunk@28305 602fd350-edb4-49c9-b593-d223f7449a82