Remove unused "verbose" field from gRPC Debug service request message

[per1234]

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• The PR has no duplicates (please search among the Pull Requests before creating one)
• The PR follows our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
  • (Generated gRPC documentation will automatically be updated to reflect this change on the docs deployment once the PR is merged)

• What kind of change does this PR introduce?

Removal of unused field from gRPC service request message field. This feature has not been implemented, so the field has no purpose.

• What is the current behavior?

The presence of a verbose field leads the user to believe this field can be used to control the verbosity of the debug command, whereas it actually has no effect.

• What is the new behavior?

Debug service gRPC interface doesn't have an unused verbose field.

• Does this PR introduce a breaking change?

This will be a breaking change for anyone using the verbose field. However, it's likely that few, if any, will be affected.

[cmaglie]

It's ok to remove a field but do not change the numeric identifier of existing fields like in:

-    string import_file = 7;
+   string import_file = 6;

this provides a better backward compatibility (well, at least the "verbose" value won't go into "import_file" when using an old client). I'll fixi it while rebasing #687

[per1234]

Thanks for the information. I was following #689 as a reference, but without really knowing what I was doing.