ARM: uniphier: drop unnecessary "bootm_low" environment define

This environment define has been here to work around the LMB
allocation error introduced by commit 9c11135ce0 ("image: fix
getenv_bootm_size() function").

It is no longer needed because the root cause was fixed by commit
0cb389dd1a ("image: fix getenv_bootm_size() function again").

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

include/configs/uniphier.h

@@ -228,7 +228,6 @@
 	"netdev=eth0\0"						\
 	"verify=n\0"						\
 	"nor_base=0x42000000\0"					\
-	"bootm_low=0x80000000\0"				\
 	"nandupdate=nand erase 0 0x00100000 &&"			\
 		"tftpboot u-boot-spl.bin &&"			\
 		"nand write $loadaddr 0 0x00010000 &&"		\