Remove unused files

[ruibaby]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

This PR removes some unused files:

1. .devcontainer/.gitpod.yml: barely used
2. .prettierignore: we've migrated to oxfmt now
3. ui/README.md: serves no purpose and is outdated

Does this PR introduce a user-facing change?

None

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
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The full list of commands accepted by this bot can be found here.

Details

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• OWNERS

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.85%. Comparing base (42c374d) to head (3936c21).
⚠️ Report is 477 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8393      +/-   ##
============================================
+ Coverage     59.55%   60.85%   +1.29%     
- Complexity     3812     3998     +186     
============================================
  Files           677      698      +21     
  Lines         23248    23876     +628     
  Branches       1500     1556      +56     
============================================
+ Hits          13846    14529     +683     
+ Misses         8764     8618     -146     
- Partials        638      729      +91     

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