Don't meddle with people's git settings

I can't see any reason for this to have been necessary in CI in the first place as we do CI runs for commits on a branch, not for a detached head.
Despite that, I've left it in the Linux and Android scripts as we don't suggest anyone runs those on their own machines.

CI/before_script.macos.sh

@@ -1,8 +1,5 @@
 #!/bin/sh -e
 
-# Silence a git warning
-git config --global advice.detachedHead false
-
 rm -fr build
 mkdir build
 cd build