avahi: remove useless 'mkdir -p' in AVAHI_INSTALL_INIT_SYSTEMD

In AVAHI_INSTALL_INIT_SYSTEMD, since we're using 'install -D' to
install /usr/lib/tmpfiles.d/avahi.conf in $(TARGET_DIR), there's no
need to have a 'mkdir -p' before that, since 'install -D' creates the
directories as needed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>