mb/aopen/dxplplusu: Remove board

This board use the LEGACY_SMP_INIT which is to be deprecated after
release 4.18.

Change-Id: Idf37ade31ddb55697df1a65062c092a0a485e175
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69114
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/arch/x86/smbios.c

@@ -382,8 +382,6 @@ static int get_socket_type(void)
 {
 	if (CONFIG(CPU_INTEL_SLOT_1))
 		return 0x08;
-	if (CONFIG(CPU_INTEL_SOCKET_MPGA604))
-		return 0x13;
 	if (CONFIG(CPU_INTEL_SOCKET_LGA775))
 		return 0x15;
 	if (CONFIG(XEON_SP_COMMON_BASE))