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?

Fixes #2393