mailbox: zynqmp-ipi: Remove redundant mbox_controller_unregister() call

The controller is registered using the device-managed function
'devm_mbox_controller_register()'. As documented in mailbox.c, this
ensures the devres framework automatically calls
mbox_controller_unregister() when device_unregister() is invoked, making
the explicit call unnecessary.

Remove redundant mbox_controller_unregister() call as
device_unregister() handles controller cleanup.

Fixes: 4981b82b ("mailbox: ZynqMP IPI mailbox controller")
Signed-off-by: Harini T <harini.t@amd.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>