refactor: remove ParcelableTrait

We're cleaning up AidlDefinedType hierarchies. ParcelableTrait is only
used in C++ backend and doesn't match with styles used in other
backends.

Bug: n/a
Test: aidl_unittests
Change-Id: Idff09ce2543e6272e2e4bde115e3a4cd6dba0dc6
6 files changed