fix: remove RAILS_ENV from mise.toml

[takahashim]

When using mise for local development environment, the RAILS_ENV environment variable was hardcoded to "development" in mise.toml.
This prevented running tests properly because RSpec could not set RAILS_ENV=test; the mise configuration always overrode it with "development".

This PR removes RAILS_ENV from mise.toml to allow test frameworks to set the appropriate environment.
Developers can still set RAILS_ENV manually when needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.75%. Comparing base (0119a5a) to head (a118528).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #493      +/-   ##
==========================================
- Coverage   92.35%   88.75%   -3.61%     
==========================================
  Files         166      166              
  Lines        4250     4250              
==========================================
- Hits         3925     3772     -153     
- Misses        325      478     +153     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[microstudi]

Actually, this mise.toml file shouldn't be here at all. Can you remove it?

[takahashim]

Thanks, I've deleted the mise.toml.