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>
diff --git a/arch/x86/lib/init_helpers.c b/arch/x86/lib/init_helpers.c
index 5e19f13..4774a9b 100644
--- a/arch/x86/lib/init_helpers.c
+++ b/arch/x86/lib/init_helpers.c
@@ -10,12 +10,6 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
-/* Get the top of usable RAM */
-__weak ulong board_get_usable_ram_top(ulong total_size)
-{
-	return gd->ram_size;
-}
-
 int init_cache_f_r(void)
 {
 #if CONFIG_IS_ENABLED(X86_32BIT_INIT) && !defined(CONFIG_HAVE_FSP) && \