gtk2 rbgtktargetlist.c: drop #if ! GTK_CHECK_VERSION(2,10,0)

Because of drop GTK+ < 2.10 support, it is unneeded.
gtk2 rbgtkradioaction.c: drop #if ! GTK_CHECK_VERSION(2,10,0)

Because of drop GTK+ < 2.10 support, it is unneeded.
gtk2 rbgdkscreen.c: drop GTK_CHECK_VERSION(2,10,0)
gtk2 rbgtknotebook.c: drop GTK_CHECK_VERSION(2,10,0)