[BUGFIX] Remove newline after ?> in AbstractController

The last commit added a newline again. This patch
removes it again.

Releases: 6.1
Fixes: #44366
Change-Id: I4bd33dc7947a64f975183f49eeffa3412f5c4d42
Reviewed-on: https://review.typo3.org/17406
Reviewed-by: Alexander Schnitzler
Tested-by: Alexander Schnitzler