Remove unused variables

Detected thanks to the RSpec/LetSetup rule after updating rubocop-rspec.

spec/shared/models/document_validations.rb

@@ -2,7 +2,6 @@ shared_examples "document validations" do |documentable_factory|
   include DocumentablesHelper
 
   let!(:document)               { build(:document, documentable_factory.to_sym) }
-  let!(:documentable)           { document.documentable }
   let!(:maxfilesize)            { max_file_size(document.documentable.class) }
   let!(:acceptedcontenttypes)   { accepted_content_types(document.documentable.class) }