Remove "Allow to mark root certificates as trusted" check box

With dfaaabb406a3 (T7350) we removed the code that hid this check box for
newer versions of GnuPG (i.e. versions 2.0.21+ that use the
no-allow-mark-trusted option instead of the allow-mark-trusted option).
We forgot to remove the check box also from the UI.

• GnuPG-bug-id: T7868