ci: remove pull_request trigger

[nytai]

🏆 Enhancements
now that #849 has merged and the actions code is in master, we can rely on pull_request_target for any changes. pull_request will fail for any PR that comes from a fork as it will not have access to secrets needed to upload the chromatic output.

more info on pull_request_target here https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows/#improvements-for-public-repository-forks

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[codecov]

Codecov Report

Merging #853 (5d8500d) into master (a6286fd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #853   +/-   ##
=======================================
  Coverage   26.55%   26.55%           
=======================================
  Files         377      377           
  Lines        8178     8178           
  Branches     1117     1117           
=======================================
  Hits         2172     2172           
  Misses       5878     5878           
  Partials      128      128           

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[rusackas]

Woohoo!