[TASK] Adapt INSTALL.txt to removed t3lib

There is no need to add symlink for t3lib.
We can strip the line from INSTALL help file.

Change-Id: Id5bb17412dc041a50d452734ab6031a7c1b041e2
Resolves: #49346
Releases: 6.2
Reviewed-on: https://review.typo3.org/21598
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn