Revert "[TASK] Additional check for valid domain in validEmail()"

Reverts #56771 which introduced brittle feature + tests to validEmail() which were beyond the scope of the function.

This reverts commit 5033a33d7b7ed13f6672db6846ad097b8120315b.

Change-Id: Ib2b25af1e65ac93c4174117dc15f2e27e5466288
Reverts: #56771
Resolves: #64064
Release: master
Reviewed-on: http://review.typo3.org/35683
Reviewed-by: Steffen Müller <typo3@t3node.com>
Tested-by: Steffen Müller <typo3@t3node.com>