[core][mod_sofia] remove ZRTP (deemed obsolete).

[unit-tests][build-system][docs] remove references to ZRTP.

conf/vanilla/vars.xml

@@ -41,7 +41,6 @@
       storage_dir
       cache_dir
       core_uuid
-      zrtp_enabled
       nat_public_addr
       nat_private_addr
       nat_type
@@ -64,12 +63,6 @@
   <X-PRE-PROCESS cmd="set" data="hold_music=local_stream://moh"/>
   <X-PRE-PROCESS cmd="set" data="use_profile=external"/>
   <X-PRE-PROCESS cmd="set" data="rtp_sdes_suites=AEAD_AES_256_GCM_8|AEAD_AES_128_GCM_8|AES_CM_256_HMAC_SHA1_80|AES_CM_192_HMAC_SHA1_80|AES_CM_128_HMAC_SHA1_80|AES_CM_256_HMAC_SHA1_32|AES_CM_192_HMAC_SHA1_32|AES_CM_128_HMAC_SHA1_32|AES_CM_128_NULL_AUTH"/>
-  <!--
-      Enable ZRTP globally you can override this on a per channel basis
-
-      http://wiki.freeswitch.org/wiki/ZRTP (on how to enable zrtp)
-  -->
-  <X-PRE-PROCESS cmd="set" data="zrtp_secure_media=true"/>
   <!--
       NOTICE: When using SRTP it's critical that you do not offer or accept
       variable bit rate codecs, doing so would leak information and possibly