Stop mentioning WikibaseLib in CREDITS

We stopped registering WikibaseLib as a separate extension in change
I37819f1fce (commit e7449c8), so Special:Version/Credits/WikibaseLib
no longer exists.

Change-Id: I241088b5a4f97c601ca00f169a9d1105cc26570a

Author: lucaswerkmeister

CREDITS

@@ -6,7 +6,6 @@ the following names for their contribution to the product.
 The following list can be found parsed under:
  * Special:Version/Credits/Wikibase_Repository
  * Special:Version/Credits/Wikibase_Client
- * Special:Version/Credits/WikibaseLib
 -->
 == Team members ==