[BUGFIX] Remove superfluous LF in install/index.php

The constant refactoring of #36889 introduced a minor issue with
not already defined constants early in install/index.php. The
constant access is not really needed anyway, so it is removed now.

Change-Id: Ic16fbb4106d8773b94bc2c4aa7ecc937e19ab4d2
Related: #36889
Resolves: #36935
Releases: 6.0
Reviewed-on: http://review.typo3.org/11060
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn