Fix offenses of naming/variable_name_spec.rb

Fix the following offenses.

```console
% bundle exec rake internal_investigation

(snip)

Offenses:

spec/rubocop/cop/naming/variable_name_spec.rb:138:5: C: Don't repeat
examples within an example group.
    it 'registers an offense for snake case in method parameter' do ...
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
spec/rubocop/cop/naming/variable_name_spec.rb:149:5: C: Don't repeat examples within an example group.
    it 'registers an offense for method arguments' do ...
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1019 files inspected, 2 offenses detected
RuboCop failed!
```

This commit removes a redundant test. The following tests are the same.

- https://github.com/bbatsov/rubocop/blob/834262bf83f00462333245547727e102736daccd/spec/rubocop/cop/naming/variable_name_spec.rb#L138-L143
- https://github.com/bbatsov/rubocop/blob/834262bf83f00462333245547727e102736daccd/spec/rubocop/cop/naming/variable_name_spec.rb#L149-L154

I left a more clearer explanation test.

spec/rubocop/cop/naming/variable_name_spec.rb

@@ -146,13 +146,6 @@ def method(funny_arg); end
       expect_no_offenses('_myLocal = 1')
     end
 
-    it 'registers an offense for method arguments' do
-      expect_offense(<<-RUBY.strip_indent)
-        def method(funny_arg); end
-                   ^^^^^^^^^ Use camelCase for variable names.
-      RUBY
-    end
-
     it 'registers an offense for default method arguments' do
       expect_offense(<<-RUBY.strip_indent)
         def foo(opt_arg = 1); end