Trac #33859: Remove sage/libs/gap/test from MANIFEST.in

This directory was removed in #27155 but `MANIFEST.in` was not cleaned
to reflect it.

It generates a non fatal warning when producing wheels for sagemath-
standard.

URL: https://trac.sagemath.org/33859
Reported by: fbissey
Ticket author(s): François Bissey
Reviewer(s): Matthias Koeppe

Author: Release Manager

src/MANIFEST.in

@@ -5,6 +5,5 @@ include pyproject.toml
 
 prune .tox
 
-graft   sage/libs/gap/test
 prune sage/ext/interpreters   # In particular, __init__.py must not be present in the distribution; or sage_setup.autogen.interpreters.rebuild will not generate the code
 prune sage_setup