[IPP] Require rcode param to only be a string type

All proxy B providers are compliant with the spec and return strings
so this code can be simplified.

Fixed: 435524190
Change-Id: Ic0e980a242df8cb53700fadb2944f61211fa67da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6925301
Commit-Queue: Zainab Rizvi <rizvis@google.com>
Reviewed-by: Andrew Williams <awillia@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1512676}