documentation: remove obsolete and broken setTabOrder() call

Exiting KDevelop after a debug session prints one or more identical runtime warnings that I have tracked to the line removed in this commit:

QWidget::setTabOrder: 'first' and 'second' must be in the same window

The removed setTabOrder() call was introduced in 81002b5f, back when mIdentifiers was inside a KToolBar. 0de9ca7b ported away from the KToolBar and removed the other setTabOrder() call introduced in the aforementioned commit.

QWidget::setTabOrder() returns early without changing anything right after printing this warning. Therefore, elimination of the warning is the only effect of this commit.