chore: remove duplicate DashboardFilter class and change imports accordingly

[amitmiran137]

SUMMARY

there was duplicate code so removed the filter.py and changed imports

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #12405 (4a0cf5d) into master (f13f2e2) will decrease coverage by 4.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #12405      +/-   ##
==========================================
- Coverage   66.88%   62.81%   -4.07%     
==========================================
  Files        1014     1013       -1     
  Lines       49528    49474      -54     
  Branches     5074     5077       +3     
==========================================
- Hits        33125    31078    -2047     
- Misses      16273    18186    +1913     
- Partials      130      210      +80     

Flag	Coverage Δ
cypress	?
javascript	60.71% <ø> (+0.01%)	⬆️
python	64.19% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/views/chart/mixin.py	100.00% <100.00%> (ø)
superset/views/dashboard/mixin.py	95.00% <100.00%> (ø)
superset-frontend/src/SqlLab/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/explore/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/dashboard/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/chart/ChartContainer.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/explore/reducers/index.js	0.00% <0.00%> (-100.00%)	⬇️
...et-frontend/src/dashboard/containers/Dashboard.jsx	0.00% <0.00%> (-100.00%)	⬇️
...et-frontend/src/filters/components/Select/types.ts	0.00% <0.00%> (-100.00%)	⬇️
...t-frontend/src/dashboard/containers/SliceAdder.jsx	0.00% <0.00%> (-100.00%)	⬇️
... and 208 more

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[villebro]

Nice catch!