Stop incorrectly returning rescue: supported

Rescue interface is not supported by ironic.drivers.modules.pxe, but all
the drivers which use the PXEDeploy interface were incorrectly mapping
it to their rescue interface as well, leading to the appearance that
these drivers supported rescue functionality.

This patch corrects this by removing the incorrect driver interface
mapping, and deleting the unused stub class PXERescue.

Change-Id: Icaa5928f18d72fcb2d26baac2a18606e8c4366c6
Closes-bug: #1294288
Merge "Stop incorrectly returning rescue: supported"