ARM64: zynqmp: Remove CONFIG_PREBOOT

CONFIG_PREBOOT variable is breaking ./test/py framework.
Remove it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/include/configs/xilinx_zynqmp.h b/include/configs/xilinx_zynqmp.h
index e36dde6..e6c38a5 100644
--- a/include/configs/xilinx_zynqmp.h
+++ b/include/configs/xilinx_zynqmp.h
@@ -159,7 +159,6 @@
 		"booti $kernel_addr - $fdt_addr\0" \
 	DFU_ALT_INFO
 
-#define CONFIG_PREBOOT		"run bootargs"
 #define CONFIG_BOOTCOMMAND	"run $modeboot"
 #define CONFIG_BOOTDELAY	3