[TASK] Remove superfluous parenthesis in sysext frontend

Change ("a" . "b") . "c" into "a" . "b" . "c" using the lex-pass tool.

Change-Id: I09b90a58a74a219e6957488a7cca7a591c27ff70
Resolves: #40881
Related: #40095
Releases: 6.0
Reviewed-on: http://review.typo3.org/14566
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel