rockchip: puma-rk3399: remove dead code

rockchip: puma-rk3399: remove dead code

CONFIG_SERIAL_TAG is not selectable for ARM64 machines. While
get_board_serial is weakly defined if ENV_VARS_UBOOT_RUNTIME_CONFIG is
defined, it is only called when CONFIG_SUPPORT_PASSING_ATAGS is defined,
which also is not selectable for ARM64 machines. Therefore this is dead
code so let's remove it.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>