Remove unused `def_node_matcher`
Remove unused `def_node_matcher`

Follow https://github.com/rubocop-hq/rubocop/pull/9231.

This commit removes unused `def_node_matcher`.
It was not removed from the generator template.

bbatsov/rubocop,bbatsov/rubocop,maxjacobson/rubocop,maxjacobson/rubocop,maxjacobson/rubocop,tejasbubane/rubocop,rrosenblum/rubocop,rrosenblum/rubocop,tejasbubane/rubocop,tejasbubane/rubocop,rrosenblum/rubocop