Remove singularity from CI images (#8945)
Remove singularity from CI images (#8945)

The singularity operator tests _have always_ used mocking, so we were
adding 700MB to our docker image for nothing.

Fixes #8774
Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,mistercrunch/airflow,apache/incubator-airflow,nathanielvarona/airflow,dhuang/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,sekikn/incubator-airflow,airbnb/airflow,mistercrunch/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,wooga/airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,mrkm4ntr/incubator-airflow,nathanielvarona/airflow,mrkm4ntr/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,wooga/airflow,DinoCow/airflow,sekikn/incubator-airflow,apache/airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,apache/airflow,bolkedebruin/airflow,lyft/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,mistercrunch/airflow,apache/incubator-airflow,apache/airflow,mistercrunch/airflow,danielvdende/incubator-airflow,DinoCow/airflow,lyft/incubator-airflow,DinoCow/airflow,DinoCow/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,apache/airflow,sekikn/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,dhuang/incubator-airflow,dhuang/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow,wooga/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,dhuang/incubator-airflow,airbnb/airflow,apache/airflow,lyft/incubator-airflow,cfei18/incubator-airflow,wooga/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow