Remove Rubocop TargetRubyVersion

We don't usually use TargetRubyVersion in other extensions:
- https://github.com/solidusio/solidus_auth_devise/blob/4a4f9f213d45b77cead29f45bf5bfd0d2021ab46/.rubocop.yml
- https://github.com/solidusio/solidus_stripe/blob/993a43db00ae15984e505a6a4641db955c0eaf1b/.rubocop.yml

Also, this was causing a lint warning as the version did not match the
minimum Ruby version listed in the gemspec. TargetRubyVersion can't be
2.5 as it's not supported by Rubocop, and thus Rubocop shouldn't ditact
the gem's required version.

.rubocop.yml

@@ -3,7 +3,6 @@ require:
 
 AllCops:
   NewCops: disable
-  TargetRubyVersion: 2.6
 
 Layout/FirstArgumentIndentation:
   EnforcedStyle: consistent