Unused fields in REST SystemProperty.java

[GCHQDeveloper314]

Describe the issue
In SystemProperty.java the fields SERVICES_PACKAGE_PREFIX and SERVICES_PACKAGE_PREFIX_DEFAULT are declared but are not used anywhere in the Gaffer codebase.

It's possible they were used at the time they were added and have since become redundant. The history behind them should be investigated and if not required these fields should be removed.

Expected situation
The codebase shouldn't contain unused code, unless there's a good reason for it.

[rohit-satya]

@GCHQDeveloper314 Please assign