MT#55283 Further rtpengine-iptables related cleanups

Follow-up for commit eea3824725, remove further files and configuration
related to rtpengine-iptables removal.

Change-Id: I3bc9a2a452bd41fb346b7479e8b00e4cdedb93a6

debian/ngcp-rtpengine-iptables.dirs

@@ -1 +0,0 @@
-${env:XTABLES_DIR}

debian/ngcp-rtpengine-iptables.install

@@ -1 +0,0 @@
-${env:XTABLES_DIR}/libxt_RTPENGINE.so ${env:XTABLES_DIR}

debian/rules

@@ -9,8 +9,6 @@ include /usr/share/dpkg/pkg-info.mk
 export DEB_VERSION_UPSTREAM
 export DEB_SOURCE
 
-export XTABLES_DIR := $(shell pkg-config xtables --variable=xtlibdir || echo /lib/xtables)
-
 ifneq (,$(filter $(DEB_BUILD_PROFILES),pkg.ngcp-rtpengine.no-transcoding))
 export with_transcoding = no
 endif