[AIRFLOW-XXX] Remove flake8 from PR template
[AIRFLOW-XXX] Remove flake8 from PR template

Since we have pre-commit and flake8 on CI there seems to be no
reason to keep this bullet point in PR template.

apache/airflow,mrkm4ntr/incubator-airflow,apache/airflow,cfei18/incubator-airflow,apache/airflow,cfei18/incubator-airflow,mrkm4ntr/incubator-airflow,spektom/incubator-airflow,sekikn/incubator-airflow,mistercrunch/airflow,mtagle/airflow,danielvdende/incubator-airflow,mtagle/airflow,Fokko/incubator-airflow,mtagle/airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,DinoCow/airflow,bolkedebruin/airflow,apache/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,airbnb/airflow,mistercrunch/airflow,spektom/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,bolkedebruin/airflow,apache/airflow,sekikn/incubator-airflow,wileeam/airflow,mrkm4ntr/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,Fokko/incubator-airflow,cfei18/incubator-airflow,apache/airflow,mistercrunch/airflow,cfei18/incubator-airflow,DinoCow/airflow,airbnb/airflow,dhuang/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,DinoCow/airflow,apache/incubator-airflow,airbnb/airflow,Acehaidrey/incubator-airflow,wileeam/airflow,Fokko/incubator-airflow,dhuang/incubator-airflow,lyft/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,sekikn/incubator-airflow,spektom/incubator-airflow,wooga/airflow,Fokko/incubator-airflow,mtagle/airflow,bolkedebruin/airflow,nathanielvarona/airflow,lyft/incubator-airflow,wileeam/airflow,airbnb/airflow,dhuang/incubator-airflow,apache/incubator-airflow,apache/incubator-airflow,sekikn/incubator-airflow,wooga/airflow,Acehaidrey/incubator-airflow,wileeam/airflow,nathanielvarona/airflow,nathanielvarona/airflow,DinoCow/airflow,wooga/airflow,spektom/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,wooga/airflow,bolkedebruin/airflow,mrkm4ntr/incubator-airflow,nathanielvarona/airflow