Simplify method to calculate document max size

Since we're only doing the convertion from bytes to megabytes in one
place, IMHO adding an extra method makes the code harder to read.

This way we don't have do include the DocumentsHelper in the specs
anymore, reducing the risk of possible method naming collisions.

spec/shared/models/document_validations.rb

@@ -1,5 +1,4 @@
 shared_examples "document validations" do |documentable_factory|
-  include DocumentsHelper
   include DocumentablesHelper
 
   let!(:document)               { build(:document, documentable_factory.to_sym) }