Properly uncheck the "new tab" box when editing a non-new-tab link.

Previously, if you had just edited a new-tab link, a subsequent edit of a non-new-tab link would mistakenly keep the checkbox checked.

props sc0ttkclark. fixes #24520.

git-svn-id: https://core.svn.wordpress.org/trunk@24419 1a063a9b-81f0-0310-95a4-ce76da25c4cd