ARM: UniPhier: remove unused checkboard() functions

Since commit 0365ffcc (generic-board: show model name in
board_init_f() too), checkboard() is invoked only when
show_board_info() fails to get the model name from Device Tree.
It never happens because UniPhier SoCs now only work with
CONFIG_OF_CONTROL and all the root nodes of UniPhier device trees
have the "model" property.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>