[TASK] Merge t3lib_userAuthGroup to t3lib_beUserAuth

t3lib_userAuthGroup is an abstract class that is only extended by
t3lib_beUserAuth. The comment states for ages that the two classes
could be merged. The patch merges all properties and methods from
t3lib_beUserAuth to t3lib_userAuthGroup and deprecates the latter.

Change-Id: Iaf12dd6b3f3aba36f5b87efc38ae60962a5f5bd9
Resolves: #39538
Releases: 6.0
Reviewed-on: http://review.typo3.org/13467
Reviewed-by: Oliver Klee
Reviewed-by: Wouter Wolters
Reviewed-by: Felix Kopp
Reviewed-by: Stefan Neufeind
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn