Remove keypair from packer build

https://gitlab.rc.uab.edu/rc/packer-openstack-hpc-image/-/blob/main/openstack/nodeimage.pkr.hcl#L20

From packer documentation, ssh_keypair_name is not require since packer will create a temporary one to use.

If specify one, ssh_private_key_file will be required which is difficult to do in a container (GitLab runner).