[Fix #6939] Remove warn for rubocop-performance (#6944)

Fixes #6939.
Fixes #6917.

This warning should not be displayed if users don't use Performance cops
as follows:

- #6845 (comment)
- #6919 (comment)

In the future, it will be better to replace it with a mechanism that
displays a warning only when the user need Performance cops.

This PR removes a warning for rubocop-performance when running `rubocop`
command.

It leaves a warning just for `gem install rubocop`.
https://github.com/rubocop-hq/rubocop/blob/v0.67.2/rubocop.gemspec#L47

exe/rubocop

@@ -9,18 +9,6 @@ require 'benchmark'
 cli = RuboCop::CLI.new
 result = 0
 
-if defined?(Bundler)
-  gemfile_lock = Bundler.read_file(Bundler.default_lockfile)
-  parser = Bundler::LockfileParser.new(gemfile_lock)
-
-  unless parser.dependencies['rubocop-performance']
-    warn Rainbow(<<-MESSAGE.strip_indent).yellow.to_s
-      [Warn] Performance Cops will be removed from RuboCop 0.68. Use rubocop-performance gem instead.
-             https://github.com/rubocop-hq/rubocop/tree/master/manual/migrate_performance_cops.md
-    MESSAGE
-  end
-end
-
 time = Benchmark.realtime do
   result = cli.run
 end