[eas-cli] Remove unused eas onboarding command

[byronkarlen]

Why

The expo.new web onboarding flow that fed this command no longer exists.

➜  ~ eas onboarding
This command can only be run as part of the onboarding process started on the Expo website. Visit https://expo.new to start a new project.
    Error: project:onboarding command failed.

This is technically a breaking change but maybe we don't need to list it as one. The command has already been failing.

How

Removed command and related query.

Test Plan

CI

[github-actions]

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[github-actions]

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File Patterns	Mentions
**/*	@douglowder

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.78%. Comparing base (cc42fdc) to head (6488906).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3993      +/-   ##
==========================================
+ Coverage   59.60%   59.78%   +0.19%     
==========================================
  Files         938      936       -2     
  Lines       41226    41040     -186     
  Branches     8678     8633      -45     
==========================================
- Hits        24568    24533      -35     
+ Misses      16564    16413     -151     
  Partials       94       94              

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