Issue #446: Remove function delete_columns()

[nikosbosse]

Description

This PR closes #446

We have a helper function delete_columns() that safely deletes columns from a data.table. Recent changes in data.table mean that you can attempt to delete a column that doesn't exist without an error. So this PR changes instances of delete_columns() with suppressWarnings(dt[, col := NULL])

Linting CI is currently failing since #511 that fixes the variable length is not yet merged.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title as follows: issue-number: PR title
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• I have built the package locally and run rebuilt docs using roxygen2.
• My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d761257) 81.78% compared to head (a7d12d1) 81.72%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #529      +/-   ##
===========================================
- Coverage    81.78%   81.72%   -0.06%     
===========================================
  Files           20       20              
  Lines         1713     1702      -11     
===========================================
- Hits          1401     1391      -10     
+ Misses         312      311       -1     

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[seabbs]

LGTM (note I've been waiting on this to pass CI checks hence the delay in review)