Remove duplicate function getOpCode().

Reviewer: @andi-bigswitch @rlane

- getOpCode() does NOT match the camel casing of the class name ArpOpcode
- getOpcode() match the camel casing of the class name ArpOpcode
- These 2 functions do the same thing
- getOpCode() (the one to be removed) does not seem to be used in Floodlight
- We cannot be sure that it is not used anywhere else though