rngd: Use upstream service file

Our custom service file is outdated and show sometimes errors during
startup.

It was created at time where the upstream recipe did not support
systemd.

BugzID: 65497

recipes-support/rng-tools/rng-tools/rngd.service

@@ -1,11 +0,0 @@
-[Unit]
-Description=Random number generator
-
-[Service]
-Type=forking
-ExecStart=/usr/sbin/rngd
-PIDFile=/var/run/rngd.pid
-KillSignal=SIGQUIT
-
-[Install]
-WantedBy=multi-user.target

recipes-support/rng-tools/rng-tools_%.bbappend

@@ -1,12 +0,0 @@
-inherit systemd
-
-FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
-
-SRC_URI_append = " file://rngd.service"
-
-SYSTEMD_SERVICE_${PN} = "rngd.service"
-
-do_install_append() {
-    install -d ${D}/lib/systemd/system
-    install -m 644 ${WORKDIR}/rngd.service ${D}/lib/systemd/system/
-}