Code improvement in commerce_order_types.install

Some code improvements ref from https://drupal.org/node/1661460#comment-6194774:

1. In commerce_order_types.install on line 11, No need to define $schema = array();. You can remove that line
2. In commerce_order_types.install file, May be you can write more description of commerce_order_types_schema() function. What that schema is for and what kind of data it stores.

Regards,
Chintan Vyas.