gitweb: Remove invalid use of str.rstrip

rstrip is not a method to remove a string suffix so use proper Python way to do it.

It fixes the computation of some gitweb origin URLs.

Related to swh/infra/sysadm-environment#5051 (closed).

swh/lister/gitweb/tests/test_lister.py

@@ -138,6 +138,10 @@ def test_lister_gitweb_get_origin_from_repo_failing(
             "https://git.shadowcat.co.uk?p=File-Slurp.git;a=summary",
             "git://git.shadowcat.co.uk/File-Slurp.git",
         ),
+        (
+            "https://git.example.org?p=baaaa;a=summary",
+            "git://git.example.org/baaaa",
+        ),
         ("https://domain.org/foobar", None),
     ],
 )