x86/iommu: remove code to fetch MSI message from remap table

x86/iommu: remove code to fetch MSI message from remap table

Remove the code to compose a MSI message based on the information from
the MSI registers and the data in the interrupt remapping table.
Since the removal of read_msi_msg and its user there's no longer a
need for such code, as the last written (untranslated) MSI message is
cached internally by Xen.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>