mtd: rawnand: gpmi: cleanup makefile

The extra gpmi_nand.o object is not needed anymore since
commit 3045f8e3 ("mtd: rawnand: gpmi: move all driver
code into single file").
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201007134533.31390-1-m.felsch@pengutronix.de

drivers/mtd/nand/raw/gpmi-nand/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
-obj-$(CONFIG_MTD_NAND_GPMI_NAND) += gpmi_nand.o
-gpmi_nand-objs += gpmi-nand.o
+obj-$(CONFIG_MTD_NAND_GPMI_NAND) += gpmi-nand.o