MIPS: xilfpga: Stop generating useless dtb.o

A dtb.o is generated from nexys4ddr.dts but this is never used since it
has been moved to mips/generic with commit b35565bb ("MIPS: generic:
Add support for MIPSfpga").

Fixes: b35565bb ("MIPS: generic: Add support for MIPSfpga")
Signed-off-by: NAlexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 4.15+
Patchwork: https://patchwork.linux-mips.org/patch/19244/Signed-off-by: NJames Hogan <jhogan@kernel.org>

arch/mips/boot/dts/xilfpga/Makefile

 # SPDX-License-Identifier: GPL-2.0
 dtb-$(CONFIG_FIT_IMAGE_FDT_XILFPGA)	+= nexys4ddr.dtb
-
-obj-y				+= $(patsubst %.dtb, %.dtb.o, $(dtb-y))