[vis] Import only world map from datamaps

[kristw]

Import only world map from datamaps instead of importing all maps by default.
Reduce the bundle size a bit. (142KB vs 114KB)

@williaster @conglei @graceguo-supercat

[williaster]

LGTM 🔥can merge once the build is fixed!

[codecov-io]

Codecov Report

Merging #6029 into master will increase coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #6029      +/-   ##
=========================================
+ Coverage   64.66%   64.7%   +0.04%     
=========================================
  Files         448     448              
  Lines       23773   23773              
  Branches     2639    2639              
=========================================
+ Hits        15373   15383      +10     
+ Misses       8387    8377      -10     
  Partials       13      13

Impacted Files	Coverage Δ
...set/assets/src/visualizations/WorldMap/WorldMap.js	0% <0%> (ø)	⬆️
superset/connectors/sqla/models.py	81.13% <0%> (+0.75%)	⬆️
superset/db_engine_specs.py	55.7% <0%> (+0.87%)	⬆️

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