gtk: Remove support of old GTK+2 (< 2.12.0).

* gtk+-2/pinentry-gtk-2.c (tooltips): Remove
(create_window): Remove use of tooltips, but use
gtk_widget_set_tooltip_text.

The commit 1590b664d88be8386a4664c2994b685187d1eb25 requires newer
GTK+.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>