remove "apparmor" build tag (Always compile AppArmor support)

The apparmor tag was introduced in a01ed80 (2014) to make cgo dependency
on libapparmor optional.

However, the cgo dependency was removed in db093f6 (2017), so it is no
longer meaningful to keep apparmor build tag.

Close #2704

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

script/release.sh

@@ -27,7 +27,7 @@ function build_project() {
 	builddir="$(dirname "$1")"
 
 	# Build with all tags enabled.
-	make -C "$root" COMMIT_NO= BUILDTAGS="seccomp selinux apparmor" static
+	make -C "$root" COMMIT_NO= BUILDTAGS="seccomp selinux" static
 	mv "$root/$project" "$1"
 }