Remove duplicate datetime precision default

We already set the default precision for all datetime columns, so there is no
need to check it in the timestamps method.

Author: sambostock

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -514,10 +514,6 @@ def check_constraint(expression, **options)
       def timestamps(**options)
         options[:null] = false if options[:null].nil?
 
-        if !options.key?(:precision) && @conn.supports_datetime_with_precision?
-          options[:precision] = 6
-        end
-
         column(:created_at, :datetime, **options)
         column(:updated_at, :datetime, **options)
       end