Drop libapparmor dependency from build docs

As of opencontainers/runc@db093f6 runc no longer depends on libapparmor
thus libapparmor-dev no longer needs to be installed to build it or
anythind that depends on it (like containerd or cri-containerd). Adjust
the documentation accordingly.

containerd/containerd#2238 did the same for containerd.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

test/build-utils.sh

@@ -34,7 +34,6 @@ apt-get update
 apt-get install -y btrfs-tools
 apt-get install -y libseccomp2/jessie-backports
 apt-get install -y libseccomp-dev/jessie-backports
-apt-get install -y libapparmor-dev
 
 # PULL_REFS is from prow.
 if [ ! -z "${PULL_REFS:-""}" ]; then