firmware: arm_scmi: drop unused `con_priv` structure member

Initially con_priv was supposedly used for transport specific data when
the SCMI driver had an abstraction to communicate with different mailbox
controllers. But after some discussions, the idea was dropped but this
variable slipped through the cracks.

This patch gets rid of this unused variable.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/common.h

@@ -86,9 +86,7 @@ struct scmi_msg {
  *	buffer for the rx path as we use for the tx path.
  * @done: completion event
  */
-
 struct scmi_xfer {
-	void *con_priv;
 	struct scmi_msg_hdr hdr;
 	struct scmi_msg tx;
 	struct scmi_msg rx;