Clean up Adjustment class

Removes info about attribute locked. It was removed in 43a3cca49180b1572e41bc3638d3ca0f0e9116d9 in favor of an event machine.  
  
Removes invalid block doc adjustment#update!. adjustment#amount is actually saved when adjustment#update! is called as can be confirmed by update_adjustment in the CalculatedAdjustments module  

Turn ruby class methods into Rails scope any reason to not keep these method as Rails scopes?