Remove unnecessary `on_numblock` method

[yasu551]

Because numbered parameters are rarely used with let.

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Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[ydah]

memo) It is true that the numbered parameter is not used for let, so it is disabled: https://github.com/rubocop/rubocop-rspec/blob/306061220137110d325d1eac4a6a328750aab700/lib/rubocop/cop/rspec/empty_line_after_final_let.rb#L26