Remove bootstrap from yarn start

[EvanBacon]

• fix Remove lerna bootstrap from yarn start and add clean command #1836
• Instead of clean, just run yarn bootstrap again.
• Removed "prebootstrap": "yarn", as learna bootstrap already runs yarn.

[codecov-io]

Codecov Report

Merging #1869 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1869   +/-   ##
=======================================
  Coverage   58.52%   58.52%           
=======================================
  Files          89       89           
  Lines        2662     2662           
  Branches      730      730           
=======================================
  Hits         1558     1558           
  Misses       1073     1073           
  Partials       31       31           

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