[TASK] PATH_thisScript is defined twice in bootstrap

In index.php AND typo3/sysext/cms/tslib/index_ts.php
(which will be included in index.php) the constant
PATH_thisScript is defined.

The definition in typo3/sysext/cms/tslib/index_ts.php
is obsolete, because this will be checked via
if(!defined(...)).

Change-Id: I3ffc0639d61f76bb6c72a96fb4fd2908e38a0268
Fixes: #35003
Releases: 6.0, 4.7
Reviewed-on: http://review.typo3.org/9739
Reviewed-by: Andy Grunwald
Tested-by: Andy Grunwald
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn