[TASK] Remove unwanted `use function` php import

Using function imports with `use` is usually not used
in the TYPO3 core. Due to an oversight in the review
for #92768 this slipped in.

This change uses the `self::assertCount()` instead of
the import function `assertCount()` to follow current
policy again.

Resolves: #101884
Related: #92768
Releases: main, 12.4
Change-Id: Ia3da8eaf1c6a00f331af83fd1efd88f125e60be5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80909


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>