Cirrus CI: Do not install coreutils on macOS. [skip appveyor]

libpcap build & test process now tolerates a missing timeout(1), so
let's keep the build environment as close to the original as possible.
This reverts commit 673a2fe.

Author: infrastation

.cirrus.yml

@@ -58,10 +58,6 @@ macos_task:
   script:
     - brew update >/dev/null
     - brew install libsmi | grep -v '%'
-    # We need coreutils so that building, checking, and installing
-    # libpcap works, as *that* requires coreutils in order to
-    # install the timeout command in macOS.
-    - brew install coreutils
     - echo '$ git clone [...] libpcap.git'
     - git -C .. clone --depth ${CIRRUS_CLONE_DEPTH} --branch=master --quiet ${LIBPCAP_GIT}
     - ./build_matrix.sh