Remove useless branch for the array predicate handler

The array predicate handler don't generate `IN` clause if the values has
only one element.

4ef77ac8de/activerecord/lib/active_record/relation/predicate_builder/array_handler.rb (L22)

Also, I've refactored to avoid extra spawn by `where.not` and directly
mutate `self.where_clause`.

activerecord/lib/active_record/relation/query_methods.rb

@@ -1146,13 +1146,9 @@ module ActiveRecord
     end
 
     def excluding!(records) # :nodoc:
-      # Treat single and multiple records differently in order to keep query
-      # clean in case of single record, ie, use != operator instead of NOT IN ().
-      if records.one?
-        where.not(primary_key => records.first.id)
-      else
-        where.not(primary_key => records)
-      end
+      predicates = [ predicate_builder[primary_key, records].invert ]
+      self.where_clause += Relation::WhereClause.new(predicates)
+      self
     end
 
     # Returns the Arel object associated with the relation.