Dropping errant RPORT required option for UDP

MSP-11652

No idea why this option was added at https://github.com/rapid7/metasploit-framework/pull/4113/files#diff-e9ac58cf40e1afdd1aecf8742cf6aaa6R24  Its addition caused Pro Discover scan to never run UDP scan and instead showed:
```
 ( UDP ) error: Msf::OptionValidateError The following options failed to validate: RPORT.
```