mtd: spear_smi: remove set but not used variable 'flash_info'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/mtd/devices/spear_smi.c: In function spear_smi_probe_config_dt:
drivers/mtd/devices/spear_smi.c:780:32: warning: variable flash_info set but not used [-Wunused-but-set-variable]

It is not used since commit 6551ab5d ("mtd:
add device-tree support to spear_smi")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>