mb/amd/mandolin: remove unused option IRQ_SLOT_COUNT

That option is only relevant if the boards selects HAVE_PIRQ_TABLE which
it doesn't.

Change-Id: I76c098c7029ed9d797f6c4fb016eaa18854fadd3
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42781
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/amd/mandolin/Kconfig

@@ -42,10 +42,6 @@ config MAX_CPUS
 	int
 	default 8
 
-config IRQ_SLOT_COUNT
-	int
-	default 11
-
 config ONBOARD_VGA_IS_PRIMARY
 	bool
 	default y