Make sure that artifacts_file is nullified after removing artifacts

## What does this MR do?
Fixes a problem that `ExpireBuildArtifactsWorker` is executed for all previously removed artifacts.

## Why was this MR needed?
Currently the `Ci::Build::erase_artifacts!` doesn't ensure that data are saved to database.
The bang at end of this method lets you believe that it should do so.
This adds a missing `save` to this method.

## What are the relevant issue numbers?
None, yet.

## CHANGELOG
Since this is regression in feature introduce in RC4 no CHANGELOG entry is needed.

cc @grzesiek 


See merge request !4741