[TASK] Remove references to oldstyle Login News (TYPO3_CONF_VARS)

Adding Login News via
$GLOBALS['TYPO3_CONF_VARS']['BE']['loginNews'] is not longer
supported but there are still references to it in the code that should be
removed.

Resolves: #44506
Releases: 6.0, 6.1

Change-Id: I2c91c3209dc4a1dc55e570e569825acb3167150e
Reviewed-on: https://review.typo3.org/17519
Reviewed-by: Mattias Nilsson
Tested-by: Mattias Nilsson
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-on: https://review.typo3.org/17524
Reviewed-by: Wouter Wolters