Move rekey test into valgrind-2.

Now that the rekey test has been optimized it's fast enough to not be in
its own valgrind test, so move it into valgrind-2, which is currently
the quickest of the others, bringing all of them to roughly the same
runtime of ~1.1 hours.

.github/workflows/c-cpp.yml

@@ -32,7 +32,6 @@ jobs:
           - { target: ubuntu-20.04, config: valgrind-2 }
           - { target: ubuntu-20.04, config: valgrind-3 }
           - { target: ubuntu-20.04, config: valgrind-4 }
-          - { target: ubuntu-20.04, config: valgrind-5 }
           - { target: ubuntu-20.04, config: valgrind-unit }
           - { target: ubuntu-20.04, config: c89 }
           - { target: ubuntu-20.04, config: clang-6.0 }