docs: update RPC error schema update process

[jakmeier]

This commit updates the README description for how to update to a new RPC error schema.

I also delete two files in near-api-js which seem to be unused. The source of truth, as far as I can tell, is in the utils package now.

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• Commit messages follow the conventional commits spec
• If this is a code change: I have written unit tests.
• If this changes code in a published package: I have run pnpm changeset to create a changeset JSON document appropriate for this change.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

I got confused as to how to update the generated files since the README is outdated and it's unclear how to update these files. But since I couldn't find out where they are used, the code only references the files in the utils package. I assume the other files are left over from previous code structures and should rather be deleted to clean things up.

Test Plan

Related issues/PRs

Related: #1358

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[jakmeier]

@race-of-sloths please, include my PR in the race

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Reviewer	Score
@andy-haynes	2

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