Build/Test Tools: Remove empty test files left behind in [62081].

The client-side media processing removal emptied these files instead of deleting them, causing PHPCS coding standards failures.

Follow-up to [62081].

Props adamsilverstein.
See #64906.


git-svn-id: https://develop.svn.wordpress.org/trunk@62082 602fd350-edb4-49c9-b593-d223f7449a82