Remove duplicate dependency development gem

This PR suppress the following warning.

```console
% 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.

rubocop.gemspec

@@ -42,5 +42,4 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency('unicode-display_width', '>= 1.4.0', '< 1.7')
 
   s.add_development_dependency('bundler', '>= 1.15.0', '< 3.0')
-  s.add_development_dependency('pry')
 end