Remove unused embedder_support::GetUserAgentMetadata variant

https://crrev.com/c/6205906 removed the only usage of the pref service
so we don't need to pass it as an argument to GetUserAgentMetadata.

Bug: 1261908
Change-Id: I4cb4dedb4ae43bb95f70b22baaf6959e4c93cba4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6964493
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Victor Tan <victortan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1522762}