Clean up some ember-metal chains code

1. The first change removes a weird instance of 
   improperly removing nodes from an array which
   would break in case there’s every a time that
   a node appears in the array more than once
   (but I’m pretty certain this is never the case)
2. Removed unnecessary .length check of 
   (str && str.length > 0)
Merge pull request #4969 from machty/clean-chain

Clean up some ember-metal chains code