Remove PlusAddressModelTypeController

Instead of having a precondition in the model type controller, it
suffices to condition the creation of the model type controller by the
relevant feature flag.

The second point mentioned in crrev.com/c/5390132 doesn't seem to apply,
since:
- We want to avoid cross data type communication.
- It suffice to disable the UI.

Bug: b/322147254
Change-Id: I8f33bab08b19f04ff03728f7b7a91dc5a996d0e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5434333


Reviewed-by: Rushan Suleymanov <rushans@google.com>
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1283846}