[DBSC] Use 'unexportable_keys' level retry mechanism for prototype

It removes the custom retrying logic for DBSC prototype to rely on the
common 'unexportable_keys' level retry mechanism instead. This is the
first step of unifying the retrying logic. In the next CL(s)
`UnexportableKeyService` API will be simplified by removing the retry
logic configuration.

OBSOLETE_HISTOGRAMS=Patterned histogram
Signin.BoundSessionCredentials.CookieRotationGenerateAssertionResult.Attempt{Count} is replaced by
Signin.BoundSessionCredentials.CookieRotationGenerateAssertionResult

Bug: 405408610
Change-Id: I588ddf122441b169ef9afab7120dc6f82421a44f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7013229
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Commit-Queue: Ernest Nguyen Hung <ernn@google.com>
Cr-Commit-Position: refs/heads/main@{#1528161}