iommu/omap: Remove omap_iommu_domain_has_cap() function

The current OMAP IOMMU ops for .domain_has_cap is a stub,
and the iommu core already returns a value of 0 if the
domain doesn't have a .domain_has_cap ops plugged in. So,
clean up this stub function.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>