[TASK] Remove constant PATH_t3lib

The directory is gone, so the constant is not needed anymore.

Resolves: #49348
Relates: #49342
Releases: 6.2
Change-Id: I0ecec0dfd985b534e3357b175d6dd1e00cc0e4bd
Reviewed-on: https://review.typo3.org/21599
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn