pdl: Fix inaccurate comment

I believe the replacing of constrained fields was never implemented.

Test: n/a.
(cherry picked from https://android-review.googlesource.com/q/commit:f91606240b648ba617065426cefa75f4507b4115)
Merged-In: I5b91d69372de77b2ad7be506eaa5daf607ff4c98
Change-Id: I5b91d69372de77b2ad7be506eaa5daf607ff4c98