[POWERPC] Remove unused CONFIG_WANT_DEVICE_TREE

CONFIG_DEVICE_TREE was the only user of CONFIG_WANT_DEVICE_TREE but
it was removed in commit id 25431333813686654907ab987fb5de10c10a16db
(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>