chore: Remove GKE mybinder build

[matthewfeickert]

Description

The GKE Binder Federation instance is being taken offline on 2023-04-28 and so requests made to gke.mybinder.org will fail. 😞 To keep the GHA workflow from failing remove the GKE build triggers.

• c.f. 2023-04-25 blog Mybinder.org reducing capacity

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* The GKE Binder Federation instance is being taken offline on
  2023-04-28 and so requests made to gke.mybinder.org will fail.
  To keep the GHA workflow from failing remove the GKE build triggers.
   - c.f. https://blog.jupyter.org/mybinder-org-reducing-capacity-c93ccfc6413f

[matthewfeickert]

I'm going to approve and merge this myself. As always, PRs approved by a single core dev can be reverted as needed by the rest of the dev team.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (3b68591) 98.30% compared to head (1f5d929) 98.30%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2189   +/-   ##
=======================================
  Coverage   98.30%   98.30%           
=======================================
  Files          69       69           
  Lines        4533     4533           
  Branches      801      801           
=======================================
  Hits         4456     4456           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	97.88% <ø> (ø)
doctest	61.12% <ø> (ø)
unittests-3.10	96.31% <ø> (ø)
unittests-3.11	96.31% <ø> (ø)
unittests-3.8	96.33% <ø> (ø)
unittests-3.9	?

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