Clean-up code in dataplane.gobra

[LinoTelschow]

As Is:

• The old parser required adding some parentheses such that the file could be parsed.
  a map look-up needs the form (map)[key] instead of the regular notation map[key]
• The original scion comments are annotated with a "(tlino) original SCION comment" suffix
• To detailed comments like:

 // (tlino) use getters to simplify verification, need to use float64 cast since gobra
// (tlino) cannot cast a numeric argument to float64

To Be:

• The extra parentheses are removed
• remove these suffixes in the original SCION comments
• simplify comments