In #23024 (merged in Sage 8.7), sage.libs.ppl was deprecated and all uses converted to using the standalone library pplpy.

We remove the deprecated library.

CC: @videlec @jplab @kliem @tscrim @fchapoton

Component: refactoring

Author: Matthias Koeppe

Branch/Commit: 8c51242

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/30595