Revert "cxl: Allow a default context to be associated with an external pci_dev"

Remove abandonned capi support for the Mellanox CX4.

This reverts commit a19bd79e.

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>