FS-7309: [mod_sofia] Allow removal of User-Agent header

In certain deployments it might be desirable to remove the User-Agent header from the SIP message.
Set the sofia profile parameter "user-agent-string" to "none" to suppress this header.

conf/insideout/sip_profiles/internal.xml

@@ -36,6 +36,8 @@
         bypass media after an attended transfer.
     -->
     <!--<param name="media-option" value="bypass-media-after-att-xfer"/>-->
+
+    <!-- Can be set to "_undef_" to remove the User-Agent header -->
     <!-- <param name="user-agent-string" value="FreeSWITCH Rocks!"/> -->
     <param name="debug" value="0"/>
     <param name="sip-trace" value="no"/>