.github/workflows/ci-linux-incremental.yml: Fixup

Author: Matthias Koeppe

.github/workflows/ci-linux-incremental.yml

@@ -83,10 +83,9 @@ jobs:
       tox_system_factors: >-
         ["ubuntu-focal",
          "ubuntu-jammy",
-         "ubuntu-mantic,
+         "ubuntu-mantic",
          "debian-bullseye",
          "debian-bookworm",
-         "ubuntu-mantic,
          "fedora-30",
          "fedora-38",
          "gentoo-python3.11",