Remove example from validation_context

Since the [`except_on` option] was added we have a better way to skip validations on certain contexts, so this example is no longer fitting.

[`except_on` option]: https://github.com/rails/rails/pull/43495/files

Author: MatheusRich

activemodel/lib/active_model/validations.rb

@@ -437,20 +437,6 @@ def validate!(context = nil)
     alias :read_attribute_for_validation :send
 
     # Returns the context when running validations.
-    #
-    # This is useful when running validations except a certain context (opposite to the +on+ option).
-    #
-    #   class Person
-    #     include ActiveModel::Validations
-    #
-    #     attr_accessor :name
-    #     validates :name, presence: true, if: -> { validation_context != :custom }
-    #   end
-    #
-    #   person = Person.new
-    #   person.valid?          #=> false
-    #   person.valid?(:new)    #=> false
-    #   person.valid?(:custom) #=> true
     def validation_context
       context_for_validation.context
     end