remove reference to performance improvement

Author: adqm

peps/pep-0798.rst

@@ -492,7 +492,7 @@ Replacing Double Loops in Comprehensions
 ----------------------------------------
 
 Replacing double loops in comprehensions avoids the need for defining and
-referencing an auxiliary variable, reducing clutter and improving performance.
+referencing an auxiliary variable, reducing clutter.
 
 * From ``importlib/resources/readers.py``::