[keyserver] Immediately delete invite link blob

Summary:
We should immediately delete a blob so that a link can be reused.

https://linear.app/comm/issue/ENG-7129/invite-link-remains-used-after-editing

Test Plan: Edit a link to a new secret and then again to the old one - both edits should succeed.

Reviewers: bartek, inka

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11269