work_92105: remove unused CONFIG_SPL_NAND_BOOT configuration option

The option is unused, SPL image gets a wanted boot device by standard
spl_boot_device() call.
Signed-off-by: NVladimir Zapolskiy <vz@mleia.com>

include/configs/work_92105.h

@@ -146,7 +146,6 @@
 /* SPL will load U-Boot from NAND offset 0x40000 */
 #define CONFIG_SPL_NAND_DRIVERS
 #define CONFIG_SPL_NAND_BASE
-#define CONFIG_SPL_NAND_BOOT
 #define CONFIG_SYS_NAND_U_BOOT_OFFS  0x00040000
 #define CONFIG_SPL_PAD_TO 0x20000
 /* U-Boot will be 0x40000 bytes, loaded and run at CONFIG_SYS_TEXT_BASE */