Refactor style_offense? method to simplify offense checks for variable types

follow up: https://github.com/rubocop/rubocop-rspec/pull/2114/files#r2398699122

Author: ydah

lib/rubocop/cop/rspec/variable_definition.rb

@@ -60,16 +60,8 @@ def correct_variable(variable)
         end
 
         def style_offense?(variable)
-          (style == :symbols && string?(variable)) ||
-            (style == :strings && symbol?(variable))
-        end
-
-        def string?(node)
-          node.str_type?
-        end
-
-        def symbol?(node)
-          node.any_sym_type?
+          (style == :symbols && variable.str_type?) ||
+            (style == :strings && variable.any_sym_type?)
         end
       end
     end