mb/purism/librem_cnl: drop MAX_CPUS from Kconfig

No need to restrict this further than the platform default,
and will be problematic with the addition of the upcoming
6C/12T Librem 14 board.

Change-Id: I1913992ec12578e1ad3bf6bf679d1a35a46d7370
Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51548
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/purism/librem_cnl/Kconfig

@@ -40,10 +40,6 @@ config CBFS_SIZE
 	default 0x800000 if BOARD_PURISM_LIBREM_MINI
 	default 0xA00000 if BOARD_PURISM_LIBREM_MINI_V2
 
-config MAX_CPUS
-	int
-	default 8
-
 config DIMM_MAX
 	int
 	default 2