[TASK] Purge onFocus occurrences from SelectSingleElement

In issue #75657 `onFocus` has been purged from `FormEngine` - purging
from JavaScript class `SelectSingleElement` has been forgotten.

Resolves: #95214
Related: #75657
Releases: master
Change-Id: I2e2be48a924bf2e9babd7f561e2ffbbe1eead3d9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71080


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>