[BUGFIX] config.compressJs enables itself

Due to a compat_mode backwards compatibility setting, the new option
config.compressJs
enables itself automatically, although not desired

Change-Id: I4ed98d778f152b120b970c75799afc7db5776607
Resolves: #31237
Releases: 4.6
Reviewed-on: http://review.typo3.org/6249
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert
Reviewed-by: Stefan Galinski
Tested-by: Stefan Galinski