Remove test_tree1_dts0 testcases

The testcases based on test_tree1_dts0.dts were added purely to test dtc's
backwards compatibility handling of the old dts-v0 format.  Since that
support has been removed, the dts has been updated to use the current
dts-v1 syntax, which makes the testcases pass, but be completely useless.

This patch removes the now obsolete testcases.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>