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.

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git-svn-id: http://core.svn.wordpress.org/trunk@28132 1a063a9b-81f0-0310-95a4-ce76da25c4cd