Remove the configuration key in the correct place

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -24,7 +24,7 @@ def postgresql_connection(config) # :nodoc:
       # Forward any unused config params to PGconn.connect.
       [:statement_limit, :encoding, :min_messages, :schema_search_path,
        :schema_order, :adapter, :pool, :checkout_timeout, :template,
-       :reaping_frequency, :insert_returning, :variables].each do |key|
+       :prepared_statements, :reaping_frequency, :insert_returning, :variables].each do |key|
         conn_params.delete key
       end
       conn_params.delete_if { |k,v| v.nil? }
@@ -458,8 +458,6 @@ def initialize(connection, logger, connection_parameters, config)
           @visitor = BindSubstitution.new self
         end
 
-        connection_parameters.delete :prepared_statements
-
         @connection_parameters, @config = connection_parameters, config
 
         # @local_tz is initialized as nil to avoid warnings when connect tries to use it