[Autofill] Remove FormStructure::form_associations_

This CL removes the voting-only FormStructure::form_associations_.
Instead, the FormAssociations object is now passed directly by the
VotesUploader to EncodeUploadRequest().

Bug: 405558390
Change-Id: I86f7986afe779444e4a88ee8fc4fd59ae9f3cfff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6424429
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1442033}