Remove macos-11.0 from the test target list.

It has been consistently failing for the past few days with a github
actions internal error.

.github/workflows/c-cpp.yml

@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         # First we test all OSes in the default configuration.
-        os: [ubuntu-20.04, ubuntu-18.04, ubuntu-16.04, macos-10.15, macos-11.0]
+        os: [ubuntu-20.04, ubuntu-18.04, ubuntu-16.04, macos-10.15]
         configs: [default]
         # Then we include any extra configs we want to test for specific VMs.
         # Valgrind slows things down quite a bit, so start them first.