[Eligibility Service] Remove logic that marks profiles for onboarding

Logic for existing onboarded profiles should be retained and no
new profiles should be set to be onboarded. Also removing the call to reset prefs on non-stable channels.

Onboarding logic will be removed in a followed up CL.

Bug: 333527273
Change-Id: I4e6d0aa75efe2a009b484033d8b2fc93943aa836
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5793553


Auto-Submit: Alvin Lam <alvingo@google.com>
Commit-Queue: Anton Maliev <amaliev@chromium.org>
Reviewed-by: Abe Boujane <boujane@google.com>
Reviewed-by: Anton Maliev <amaliev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1343019}