[SQLA] Improving performance by disabling editing Associated Charts

[john-bodley]

This PR helps to improve the performance of table CRUD view by removing the slices (Associated Charts) column from the edit view (it's still present for show). For reference for our datasource the page load time reduced from ~ 45 s to ~ 7 s.

As the number of slices grows the performance of this page is severely impacted as FAB needs to pre-fetch all the slices which are searchable in jQuery. Further it seems having the ability to either add/remove slices associated with a datasource via the CRUD view is somewhat atypical as it could result in ill-defined charts and dangling charts respectively.

This significantly improves the performance when view the Detail, List Columns, or List Metrics columns. Note however the table drop-down is still susceptible to the same issue, however this field is required in order to associate a new column/metric with the datasource.

to: @graceguo-supercat @michellethomas @mistercrunch @timifasubaa

[codecov-io]

Codecov Report

Merging #5136 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #5136   +/-   ##
=======================================
  Coverage   77.51%   77.51%           
=======================================
  Files          44       44           
  Lines        8734     8734           
=======================================
  Hits         6770     6770           
  Misses       1964     1964

Impacted Files	Coverage Δ
superset/connectors/sqla/views.py	70.47% <100%> (ø)	⬆️

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[mistercrunch]

LGTM