Miscvizcleanups (#3476)
Miscvizcleanups (#3476)

* viz: remove unused countries import

* viz: simplify code in BaseViz.query_obj

alanmcruickshank/superset-dev,alanmcruickshank/superset-dev,FrederichCheng/incubator-superset,apache/incubator-superset,dmigo/incubator-superset,dmigo/incubator-superset,apache/incubator-superset,airbnb/superset,alanmcruickshank/superset-dev,zhouyao1994/incubator-superset,FrederichCheng/incubator-superset,mistercrunch/panoramix,timifasubaa/incubator-superset,dmigo/incubator-superset,apache/incubator-superset,zhouyao1994/incubator-superset,airbnb/caravel,mistercrunch/panoramix,apache/incubator-superset,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,airbnb/superset,apache/incubator-superset,timifasubaa/incubator-superset,dmigo/incubator-superset,zhouyao1994/incubator-superset,FrederichCheng/incubator-superset,airbnb/superset,airbnb/caravel,mistercrunch/panoramix,mistercrunch/panoramix,airbnb/superset,timifasubaa/incubator-superset,airbnb/caravel,FrederichCheng/incubator-superset,timifasubaa/incubator-superset,alanmcruickshank/superset-dev,airbnb/caravel