Remove useless error handling in prep_resize

The MigrationPreCheckError handling in prep_resize was
added in commit e24f333cd5
which at the same time added code to the libvirt driver's
migrate_disk_and_power_off method to raise MigrationPreCheckError.

However, migrate_disk_and_power_off is only called from the
ComputeManager.resize_instance method, which is called from
the ComputeManager._prep_resize method but via an RPC cast.

Because of the RPC cast from the destination node _prep_resize
to the source node resize_instance, the MigrationPreCheckError
from the migrate_disk_and_power_off method on the source node
is not going to come back and be raised out of _prep_resize,
thus making the error handling in prep_resize dead (and confusing)
code.

This simply removes it.

Change-Id: I60894e9c6f03b20dc59be8d9b01808da3980f4b1

nova/compute/manager.py

@@ -3794,16 +3794,6 @@ class ComputeManager(manager.Manager):
                 self._prep_resize(context, image, instance,
                                   instance_type, filter_properties,
                                   node, clean_shutdown)
-            # NOTE(dgenin): This is thrown in LibvirtDriver when the
-            #               instance to be migrated is backed by LVM.
-            #               Remove when LVM migration is implemented.
-            except exception.MigrationPreCheckError:
-                # TODO(mriedem): How is it even possible to get here?
-                # _prep_resize does not call the driver. The resize_instance
-                # method does, but we RPC cast to the source node to do that
-                # so we shouldn't even get this exception...
-                failed = True
-                raise
             except Exception:
                 failed = True
                 # try to re-schedule the resize elsewhere: