mailbox: ti-msgmgr: Get rid of unused structure members

Though q_proxies and q_slices do describe the hardware configuration,
they are not necessary for operation given that the values are
always default. Hence drop the same.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>