It never has been used in core since it was introduced in 2011
by MDL-22112, because the ouwiki_diff_html() is used unconditionally
for all wiki formats (html, creole or nwiki).
So, removing it to avoid any current (php80) or future incidence
with it.
Differences can be checked with git diff --ignore-all-space and
that will show that the only non-whitespace change is in a test
in the git lib/tests/html2text_test.php file. In that case the
whitespace has been replaced by a quoted string (of whitespaces)
keeping the test 100% equivalent and workarrounding the whitespace.
