CONFIG_DEVICE_TREE was the only user of CONFIG_WANT_DEVICE_TREE but
it was removed in commit id 25431333
(bootwrapper: Build multiple cuImages).

This removes CONFIG_WANT_DEVICE_TREE from Kconfig and the defconfigs.
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>