[keyserver] move mark_prekey_as_published calls inside of publishPrekeysToIdentity

Summary:
we always call mark_prekey_as_published after calling publishPrekeysToIdentity (except in an old migration, which won't be run again), so we can just move the call inside the function.

Depends on D12442

Test Plan: generated new unpublished prekeys and called validateAndUploadAccountPrekeys, confirmed that the prekeys were uploaded to identity and marked as published

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12443