Some UEFI systems (observed with a Mac Pro) do not provide a loaded
image device path protocol.  We don't currently use the loaded image
device path protocol for anything beyond printing a debug message, so
simply remove the code which attempts to fetch it.

Reported-by: Matt Woodward <pxematt@woodwardcc.com>
Tested-by: Matt Woodward <pxematt@woodwardcc.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>