XNAT-4363 Fixed default for pet mr.

src/main/resources/META-INF/xnat/spawner/site-admin-elements.yaml

@@ -1020,9 +1020,6 @@ petMr:
             description: >
                 Should data generated by PET-MR scanners be created as a single PET/MR imaging session, created as a single PET imaging session, or separated into PET and MR sessions?
             options:
-              delete:
-                label: Use the system default
-                value: delete
               petmr:
                 label: Create as PET/MR session
                 value: petmr