Switch from unlink to wp_delete_file

[dkotter]

What?

Closes #160

Fixes failing Plugin Check tests

Why?

We recently updated our plugin check tests and we now have a new failure. This PR addresses that

How?

Switch from using unlink to using wp_delete_file (which uses unlink under the hood)

Testing Instructions

Not really needed but if desired, can follow testing instructions in #134

[github-actions]

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Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: juanmaguitar <juanmaguitar@git.wordpress.org>

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.89%. Comparing base (7dbbc44) to head (2ab5a97).
⚠️ Report is 35 commits behind head on develop.

Files with missing lines	Patch %	Lines
includes/Abilities/Image/Import_Base64_Image.php	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #163   +/-   ##
==========================================
  Coverage      46.89%   46.89%           
  Complexity       208      208           
==========================================
  Files             19       19           
  Lines           1271     1271           
==========================================
  Hits             596      596           
  Misses           675      675           

Flag	Coverage Δ
unit	46.89% <0.00%> (ø)

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