MNT Fix js lint errors

[emteknetnz]

Issue silverstripe/.github#99

I did a merge-up from 2.0 => 2 though there were some yarn lint failures which I choose to ignore for the sake of completing the merge-up without having to commit changes without a PR. I've fixed the yarn lint errors in this PR

The yarn lint errors were

vendor/silverstripe/asset-admin/client/src/containers/AssetAdmin/AssetAdmin.js
  686:13  error  'archiveFiles' is assigned a value but never used  no-unused-vars

vendor/silverstripe/asset-admin/client/src/containers/BulkDeleteConfirmation/BulkDeleteMessage.js
  78:5  error  Prefer fragment shorthand over React.Fragment  react/jsx-fragments

vendor/silverstripe/asset-admin/client/src/containers/BulkDeleteConfirmation/DeletionModal.js
  22:1  error  Expected indentation of 8 spaces but found 10  indent

Note this PR is only fixing lint issues, there is a broken behat test on this which will be an existing failure

[GuySartorelli]

there is a broken behat test on this which will be an existing failure

Is there a PR to fix that?

[GuySartorelli]

Looks good. I was going to merge up and rerun the behat tests but the auto-merger will fail because of the linting issues and I can't be bothered merging up manually for unrelated behat failures. So, good to go.