src: hooks: remove urlhook feature

This feature is mostly a relicate from SFLPhone and introduced a remote
attack vector abusing the system() function weaknesses.

Provided that "sipEnabled" parameter is on in the remote target's
configuration, a malicious peer calling that remote target could
send SIP messages with a crafted "X-ring-url" string in order to
execute arbitrary shell commands on the target.

Header entry "X-ring-url" content is actually consumed by UrlHook
as arguments for the "x-www-browser" command executed using system().

By adding a shell escape sequence to circumvent existing arguments
sanitizing attempts, the malicious peer could execute any shell command
under remote peer user's identity and access sensitive information
available using its privileges.

Remove that feature altogether and enforce users that are relying on
that feature to migrate to Jami "plugins", which are more suitable
for introducing custom Jami behaviors.

Change-Id: I1d6d07771e2b5a7c7f2cb8fc838821106c0a6708