mailbox: omap: Remove unused omap_mbox_{enable,disable}_irq() functions

These function are not used, remove these here.

While here, remove the leading _ from the driver internal functions that
do the same thing as the functions removed.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>