mailbox/omap: remove OMAP1 mailbox driver

There are no existing users for OMAP1 mailbox driver
in kernel. Commit ab6f7751 "Removing dead OMAP_DSP"
has cleaned up all the dead code related to the only
possible user, including the creation of the mailbox
platform device.

Remove this stale driver so that the OMAP mailbox
driver can be simplified and streamlined better for
converting to mailbox framework.

Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tony Lindgren <tony@atomide.com>