Remove failing `is_textdomain_loaded()` test.

The removed test has been added as a patch to the original ticket, #21319.

Props MikeHansenMe.
See #30284.
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git-svn-id: http://core.svn.wordpress.org/trunk@30510 1a063a9b-81f0-0310-95a4-ce76da25c4cd