Revert "cxl: Add cxl_slot_is_supported API"

Remove abandonned capi support for the Mellanox CX4.

This reverts commit 4e56f858

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Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>