RPORT is required by UDPScanner; deregister instead

This is the right approach for #4265, IMO.  

Without this (or #4265), this fails:

```
resource (/tmp/u.rc)> use auxiliary/scanner/discovery/udp_sweep
resource (/tmp/u.rc)> set RHOSTS 10.0.1.1
RHOSTS => 10.0.1.1
resource (/tmp/u.rc)> run
[-] Auxiliary failed: Msf::OptionValidateError The following options failed to validate: RPORT.
```

With this and #4265, this succeeds but without removing the ```RPORT``` requirement from ```UDPScanner```:

```
resource (/tmp/u.rc)> use auxiliary/scanner/discovery/udp_sweep
resource (/tmp/u.rc)> set RHOSTS 10.0.1.1
RHOSTS => 10.0.1.1
resource (/tmp/u.rc)> run
[*] Sending 13 probes to 10.0.1.1->10.0.1.1 (1 hosts)
[*] Discovered SNMP on 10.0.1.1:161 (Apple AirPort - Apple Inc., 2006-2012. All rights Reserved.)
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
```