[keyserver] Skip early exit in deleteThread if ignorePermissions

Summary:
When I ran the migration in D9230 on production, it succeeded but didn't delete the threads. After some testing I realized there was an error in D9229, which didn't come up in my testing because I was testing in a community that commbot happened to be part of in my local environment.

The issue is that the "early exit" here will trigger if there is no permissions blob. In the case of ignorePermissions, we want the operation to go through even if there is no permissions blob for the script viewer's user (commbot).

Test Plan: I applied this patch in production, and then re-ran the migration. The bad threads were finally deleted

Reviewers: atul, rohan, inka

Reviewed By: rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D9234