Remove Range#minmax refinement (#307)

It was added for a bug in Ruby <= 2.7
The minimum version now is 2.7 so it is not needed anymore.

#151

```
irb(main):001:0> RUBY_VERSION
=> "2.6.10"
irb(main):002:0> Range.instance_method(:minmax).owner != ::Range
=> true

irb(main):002> RUBY_VERSION
=> "2.7.8"
irb(main):003> Range.instance_method(:minmax).owner != ::Range
=> false
```

lib/rubocop/ast.rb

@@ -5,7 +5,6 @@
 require 'set'
 
 require_relative 'ast/ext/range'
-require_relative 'ast/ext/range_min_max'
 require_relative 'ast/node_pattern/method_definer'
 require_relative 'ast/node_pattern'
 require_relative 'ast/node/mixin/descendence'

lib/rubocop/ast/node_pattern/node.rb

@@ -7,7 +7,6 @@ class NodePattern
       class Node < ::Parser::AST::Node
         extend Forwardable
         include ::RuboCop::AST::Descendence
-        using Ext::RangeMinMax
 
         MATCHES_WITHIN_SET = %i[symbol number string].to_set.freeze
         private_constant :MATCHES_WITHIN_SET