migration: simplify removal of local volumes and get rid of self->{volumes}

migration: simplify removal of local volumes and get rid of self->{volumes}

This also changes the behavior to remove the local copies of offline migrated
volumes only after the migration has finished successfully (this is relevant
for mixed settings, e.g. online migration with unused/vmstate disks).

local_volumes contains both, the volumes previously in $self->{volumes}
and the volumes in $self->{online_local_volumes}, and hence is the place
to look for which volumes we need to remove. Of course, replicated
volumes still need to be skipped.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>