Added Order helpers to clear adjustments

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

Partially related to Sylius/Sylius#1321 - adds extra helpers to clear adjustments, just as items can be cleared.