Remove cache clearing workaround from test_get_terms_exclude_tree()

[SergeyBiryukov]

test_get_terms_exclude_tree() has a workaround for clearing taxonomy cache:
tags/3.8.1/tests/phpunit/tests/term/getTerms.php#L168​ (added in [25933]).

It's no longer needed after the changes in #14485 and #25711.

[wonderboymusic]

In 27459:

Remove a cache-busting hack from Tests_Term_getTerms::test_get_terms_exclude_tree(). It is no longer necessary after #14485 and #25711.

See [27300].
Props SergeyBiryukov.
Fixes #27313.