Remove duplicate dependency development gem

[koic]

This PR suppress the following warning.

% cd path/to/rubocop
% bundle install
Your Gemfile lists the gem pry (>= 0) more than once.
You should probably keep only one of them.
Remove any duplicate entries and specify the gem only once.
While it's not a problem now, it could cause errors if you change the
version of one of them later.
Using rake 12.3.3
Using public_suffix 4.0.3

(snip)

pry gem is already in the Gemfile.
This looks like it was accidentally added in #7663.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Thanks!