Login and Registration: Remove duplicate `<label>` tag from `show_blog_form()` in `wp-signup.php`.

Props vaishalipanchal, audrasjb.
Fixes #49174.
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git-svn-id: https://core.svn.wordpress.org/trunk@46874 1a063a9b-81f0-0310-95a4-ce76da25c4cd