docs: remove Gitter link from README (#8549)
docs: remove Gitter link from README (#8549)


airbnb/superset,zhouyao1994/incubator-superset,airbnb/superset,airbnb/caravel,mistercrunch/panoramix,airbnb/caravel,zhouyao1994/incubator-superset,apache/incubator-superset,airbnb/superset,apache/incubator-superset,apache/incubator-superset,airbnb/caravel,zhouyao1994/incubator-superset,airbnb/superset,mistercrunch/panoramix,apache/incubator-superset,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,mistercrunch/panoramix,mistercrunch/panoramix,apache/incubator-superset,airbnb/caravel