x86: Drop weak version board_get_usable_ram_top()

Every x86 platform provides board_get_usable_ram_top(), hence there
is no need to provide a weak version board_get_usable_ram_top(), not
to mention there is another weak version board_get_usable_ram_top()
in common/board_f.c.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Aiden Park <aiden.park@intel.com>