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

- https://github.com/rubocop-hq/rubocop/pull/6845#issuecomment-481810841
- https://github.com/rubocop-hq/rubocop/pull/6919#issuecomment-483005401

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
jfelchner/rubocop,meganemura/rubocop,vergenzt/rubocop,mikegee/rubocop,vergenzt/rubocop,panthomakos/rubocop,deivid-rodriguez/rubocop,jmks/rubocop,vergenzt/rubocop,mikegee/rubocop,maxjacobson/rubocop,jfelchner/rubocop,tdeo/rubocop,deivid-rodriguez/rubocop,maxjacobson/rubocop,tdeo/rubocop,bbatsov/rubocop,tejasbubane/rubocop,meganemura/rubocop,jmks/rubocop,jfelchner/rubocop,tejasbubane/rubocop,jmks/rubocop,panthomakos/rubocop,rrosenblum/rubocop,deivid-rodriguez/rubocop,rrosenblum/rubocop,rrosenblum/rubocop,smakagon/rubocop,smakagon/rubocop,panthomakos/rubocop,tdeo/rubocop,bbatsov/rubocop,meganemura/rubocop,mikegee/rubocop,tejasbubane/rubocop,maxjacobson/rubocop