[TASK] Remove Environment Check checkMailCapabilities

The check is removed due to providing sometimes inaccurate error status
based on php.ini settings.
For now the real mail test in Test Setup got some hints in case of
missing test email. Later on the auto configuration tool (that not yet
exists) will take over from here.

Resolves: #49357
Releases: 6.2, 6.1
Change-Id: I290cca3fa4653b3a5310b9f27c2b125d9bfeb460
Reviewed-on: https://review.typo3.org/21607
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe