Skylake boards: Drop setting useless IRQ_SLOT_COUNT

The `IRQ_SLOT_COUNT` value is only meaningful when generating a PIRQ
table. None of these boards do it, so specifying this value achieves
absolutely nothing. Drop it to prevent further useless copy-pasting.

Change-Id: I2d63b850c03fc1471c0eef180e8b621311b2c336
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58362
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

src/mainboard/libretrend/lt1000/Kconfig

@@ -12,10 +12,6 @@ config BOARD_SPECIFIC_OPTIONS
 	select SPD_READ_BY_WORD
 	select SUPERIO_ITE_IT8786E
 
-config IRQ_SLOT_COUNT
-	int
-	default 18
-
 config MAINBOARD_PART_NUMBER
 	default "LT1000"