Follow-up: Add wp_is_valid_utf8() for normalizing UTF-8 checks.

When the `seemsUtf8` test file was removed, the file was left in subversion as an empty file. This patch removes the file.

Discussed in https://core.trac.wordpress.org/ticket/38044

Follow-up to [60630].

Props dmsnell, johnbillion.
See #38044.


git-svn-id: https://develop.svn.wordpress.org/trunk@60631 602fd350-edb4-49c9-b593-d223f7449a82

Author: dmsnell