Modify/Cleanup template files after PAM50_cord is updated

[jcohenadad]

After modifying the spinal cord segmentation in #21, we need to consider updating/removing:

• PAM50_levels_continuous.nii.gz,
• PAM50_csf.nii.gz,
• PAM50_centerline.nii.gz
• PAM50_label_body.nii.gz
  • related to Confusing existence of PAM50_label_body.nii.gz spinalcordtoolbox#4229
• PAM50_label_spinal_levels.nii.gz
  • Is PAM50_label_spinal_levels.nii.gz used? #28
• PAM50_label_disc.nii.gz vs. PAM50_label_discPosterior.nii.gz

As per #21 (comment)