setobject: remove out of date docstring info (GH-118048)

Objects/setobject.c

diff --git a/Objects/setobject.c b/Objects/setobject.c
index d5030ce..0d88f4f 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -2,7 +2,7 @@
 /* set object implementation
 
    Written and maintained by Raymond D. Hettinger <python@rcn.com>
-   Derived from Lib/sets.py and Objects/dictobject.c.
+   Derived from Objects/dictobject.c.
 
    The basic lookup function used by all operations.
    This is based on Algorithm D from Knuth Vol. 3, Sec. 6.4.