Remove schema-next file and requirements to update it

[lmolkova]

Removes requirement to update schema-next.yaml when making breaking changes.

See:

• Fix typo in schemas for messaging attribute changes #1595 - we had a typo and there was no tooling to catch it. Since schema files are immutable, there is no way to fix it now
• Rename db.name to db.namespace and generalize it #911 - we renamed attributes only on spans (but attributes apply to metrics too) and now there is no way to fix it
• many breaking changes can't be expressed:
  • Schema transformations should be able to convert metric units opentelemetry-specification#3729
  • Schema transformation: type mismatch/conversion should be specified opentelemetry-specification#3992
  • Schema translation for http.flavor -> net.app.protocol.name/version opentelemetry-specification#3162
• the schema processor has been in noop state for the past 2+ years https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/75ceb5598807a8bb5c45d4d4f734ce2f6c88d25c/processor/schemaprocessor/transformer.go#L43
• Conditions to rename attributes, metrics, and events in stable semconv opentelemetry-specification#3513
• Tooling: Can we come up with another way to edit CHANGELOG and schema-next.yaml? #617

Let's admit that the process is not working and stop updating schema files manually and invest into automated process - #1426 and open-telemetry/weaver#186

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]