rusty-gd: Use entire slice during packet parsing

The field offsets for the child packets are relative to the parent
packet so we do not need to use specify a starting offset when calling
the child packet's parse method.

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost

Change-Id: I472f3365259c653143a973c4412d685c20f62808