cpu/x86/smm: Drop unused label

To avoid confusion drop the label smm_handler_start, since it's unused
and the same name is also used in the permanent handler as C entry point
(see smm_module_handler.c).

Change-Id: I22bb5d378e8848d526f897cb2de70c0221827d0d
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87554
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Shuo Liu <shuo.liu@intel.com>

src/cpu/x86/smm/smm_stub.S

@@ -48,7 +48,6 @@ fallback_stack_top:
 .code16
 .global _start
 _start:
-smm_handler_start:
 #if CONFIG(SMM_LAPIC_REMAP_MITIGATION)
 	/* Check if the LAPIC register block overlaps with the stub.
 	 * This block needs to work without data accesses because they