ARM: uniphier: remove kernel parameter settings from environment

Currently, console=ttyS0 is hard-coded in CONFIG_EXTRA_ENV_SETTINGS
and it replaces the bootargs in the chosen node of the device tree
passed to the kernel.  This is not preferable because I am going to
add some boards whose console is not ttyS0.

Drop bootargs settings from U-Boot's environment and use the one in
device tree by default.

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