clean up legacy stuff

[dsokal]

Checklist

• I've added an entry to CHANGELOG.md if necessary.
• I've tagged the changelog entry with [EAS BUILD API] if it's a part of a breaking change to EAS Build API (only possible when updating @expo/eas-build-job package).

Why

• experimental.npmToken in credentials.json is a legacy way of providing the token to EAS Build. EAS Secrets should be used instead. This is what our docs are suggesting to do - https://docs.expo.io/build-reference/how-tos/#how-to-use-private-package-repositories
• --allow-experimental flag for eas build:configure has no effect now (it's not related to the previous point).

How

• I updated the credentials.json schema and removed the experimental.npmToken field.
• I removed --allow-experimental flag from eas build:configure.

Test Plan

None

[github-actions]

Size Change: -1.52 kB (0%)

Total Size: 37.5 MB

Filename	Size	Change
./packages/eas-cli/dist/eas-linux-x64.tar.gz	37.5 MB	-1.52 kB (0%)

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