[PWA] Remove ghost policy app migration code

This CL removes the migration feature flag, pref field and the code
itself that was added to fix ghost apps created due to policy installs
of URLs that loaded erroneously. This should be a no-op now, since
the code was added in M115 and scheduled to be deleted in M121 anyway,
and only ran once per profile startup.

CL that added this code: crrev.com/c/4825585.

Bug: 1440946
Change-Id: Ia87687c1466657f0066daa9b66204ea3a84ec3a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401584


Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Dibyajyoti Pal <dibyapal@chromium.org>
Reviewed-by: Glen Robertson <glenrob@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279827}