res_pjsip_endpoint_identifier_ip.c: Remove unnecessary requirement.

The requirement that "ip" must be in the endpoint identify_by list to
allow the type=identify method to identify the endpoint is not necessary.
The "ip" identifier method can match one and only one endpoint.  To even
work, the "ip" identifier method configuration must explicitly specify the
identified endpoint.  Therefore, why bother configuring the type=identify
identifier in the first place?  The requirement only adds the potential
for configuration errors for no benefit.  Even worse, those configuration
errors cannot be detected when the configuration loads.  The requirement
was introduced with the ASTERISK_27206 patch.

* Remove the code change that enforces the requiremnt.  Listing the "ip"
method in the identify_by value is simply documentation.

Change-Id: Ia057f92a33fb5d9f51dc5d5692e3d5ee1a6f2c11