of: unittest: let dtc generate __local_fixups__

Remove the manually added __local_fixups__ because dtc can now generate
them. This also fixes a new warning in the process:

drivers/of/unittest-data/testcases.dtb: Warning (interrupts_extended_property): Could not get phandle node for /__local_fixups__/testcase-data/interrupts/interrupts-extended0:interrupts-extended(cell 3)

Signed-off-by: Rob Herring <robh@kernel.org>