Remove unused methods on ActiveStorage::Variant

These methods were added in b221a4d
But they don't seem to be used by Rails internally or have any tests, so
I assume they were added by accident?
As they both seem to be marked as :nodoc: on ActiveStorage::Blob, we can
remove them without a deprecation warning.

If we decide to keep these methods, they should be added to
ActiveStorage::VariantWithRecord as well. No one complaining about there
methods missing on ActiveStorage::VariantWithRecord is another reason
these methods aren't used.

activestorage/app/models/active_storage/variant.rb

@@ -91,12 +91,6 @@ def filename
     ActiveStorage::Filename.new "#{blob.filename.base}.#{variation.format.downcase}"
   end
 
-  alias_method :content_type_for_serving, :content_type
-
-  def forced_disposition_for_serving # :nodoc:
-    nil
-  end
-
   # Returns the receiving variant. Allows ActiveStorage::Variant and ActiveStorage::Preview instances to be used interchangeably.
   def image
     self