KDE4: Drop Qt 4.9 support

I don't think there will ever be any 4.9 release, so drop all the
unused code.

(cherry picked from commit 7dd973344a8e2c09ac52aa02739b6b921f6df87e)

Change-Id: Ieca21e5d62d478dcde26f812e246d27029027bb5
Reviewed-on: https://gerrit.libreoffice.org/8706
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>