[TASK] Remove useless test in GeneralUtilityTest

This change removes a useless test, which only tests PHP
native `htmlspecialchars()` and `htmlspecialchars_decode()`
functionality.

The test did previously test `GU:htmlspecialchars_decode()`,
which has replaced by PHP native `htmlspecialchars_decode()`
in #54777.

Resolves: #101215
Related: #54777
Releases: main, 12.4
Signed-off-by: Torben Hansen <derhansen@gmail.com>
Change-Id: I11b68e74eb55ed160931353fb0f25c0340732c67
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79538


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>