remove unused CaseTypeMixin method
remove unused CaseTypeMixin method

- unused
- referenced case/all_cases, so one fewer reference

dimagi/commcare-hq,puttarajubr/commcare-hq,dimagi/commcare-hq,qedsoftware/commcare-hq,puttarajubr/commcare-hq,puttarajubr/commcare-hq,qedsoftware/commcare-hq,qedsoftware/commcare-hq,puttarajubr/commcare-hq,qedsoftware/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,qedsoftware/commcare-hq,dimagi/commcare-hq