ERP5Type: don't set portal_type on instance

It's enough to set on the class and if we don't have it on instances, it
saves a few bytes in pickles
2 jobs for feat/portal_type_attribute in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner2

00:38:19

failed ERP5.UnitTest-TestRunner2

00:58:34