pdl: Remove FieldPath from linter

Used to provide additional information for lint diagnostics,
but was unused in the end. Removing to cleanup the scope
since it is now public.

Test: atest --host pdl_python_generator_test
(cherry picked from https://android-review.googlesource.com/q/commit:89f66b6ff6d477e91ae0f86b63edf7e9adca9c89)
Merged-In: I7a27c03f0c03de9d95da870b34ace1edabe7be13
Change-Id: I7a27c03f0c03de9d95da870b34ace1edabe7be13