Task 4291: Remove Credit memo

"When a user is looking for a serial number the system in some cases
return a credit memo instead of a invoice. Please exclude the Credit
Memos from that search.

Steps:
- RMA
- Check Warranty
- Add a serial number like: 352165050102032 http://d.pr/i/2C92

The invoice 27005 is a credit memo.

We can also duplicate this on:
RMA
Receive RMA http://d.pr/i/s8hq"