selinux: Don't include unused reference count in verbose logging

selinux: Don't include unused reference count in verbose logging

libselinux 3.8 includes an API break in which the `refcnt` field of
struct security_id_t (originally intended to be a reference count, but
in practice always initialized to 1 and never modified) was renamed and
repurposed as an `id` field. This caused a build failure if dbus was
compiled with both SELinux support and verbose mode, for example in the
instrumented debug build that Debian includes in the `dbus-tests` package.

This particular piece of debug logging has little value, so just
remove it.

Reference: https://github.com/SELinuxProject/selinux/commit/e5fd7b078fb8eb0b15eb5beaccd0e6a07ec26758
Bug-Debian: https://bugs.debian.org/1096212
Signed-off-by: Simon McVittie <smcv@collabora.com>