Fix groupable only columns populating in filters

[1AB9502]

This pull requests fixes an issue where columns defined as groupable only, show up in the filters as well

Fixes this issue #6949

[jdbertron]

Yay.

[codecov-io]

Codecov Report

Merging #6993 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #6993   +/-   ##
=======================================
  Coverage   64.46%   64.46%           
=======================================
  Files         421      421           
  Lines       20521    20521           
  Branches     2245     2245           
=======================================
  Hits        13229    13229           
  Misses       7165     7165           
  Partials      127      127

Impacted Files	Coverage Δ
superset/assets/src/explore/controls.jsx	42.4% <100%> (ø)	⬆️

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[mistercrunch]

This LGTM and is the intended behavior, though leaving open for a moment to leave others the opportunity to chime in.