Remove legacy traits for ContextCreationAttribs

Struct is not send over legacy IPC anymore, last use was removed a year
ago in https://crrev.com/c/4567465

Change-Id: Ifc151aaa55c1cc215f4ee60a0b353f38eaa9d3ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5615277


Reviewed-by: Joe Mason <joenotcharles@google.com>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313060}