rename delete_all_records to delete_or_nullify_all_records

Rename delete_all_records because this name better describes
what the method is doing. We can then remove :all from the
hm:t version and pull out the unoptimized call to load_target
in delete_records and pass it directly.

activerecord/lib/active_record/associations/collection_association.rb

@@ -194,7 +194,7 @@ def delete_all(dependent = nil)
                       options[:dependent]
                     end
 
-        delete_all_records(dependent).tap do
+        delete_or_nullify_all_records(dependent).tap do
           reset
           loaded!
         end

activerecord/lib/active_record/associations/has_many_association.rb

@@ -113,7 +113,7 @@ def delete_count(method, scope)
           end
         end
 
-        def delete_all_records(method)
+        def delete_or_nullify_all_records(method)
           count = delete_count(method, self.scope)
           update_counter(-count)
         end

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -130,17 +130,13 @@ def update_through_counter?(method)
           end
         end
 
-        def delete_all_records(method)
-          delete_records(:all, method)
+        def delete_or_nullify_all_records(method)
+          delete_records(load_target, method)
         end
 
         def delete_records(records, method)
           ensure_not_nested
 
-          # This is unoptimised; it will load all the target records
-          # even when we just want to delete everything.
-          records = load_target if records == :all
-
           scope = through_association.scope
           scope.where! construct_join_attributes(*records)