[lib] Remove support for ServerThreadInfo from useKeyserverAdmin

Summary:
In D10107 I encountered a scenario where flow wasn't able to type check ServerLegacyRoleInfo as part of a ServerThreadInfo being constructed.

To be safe, I decided to look at all instances of ServerThreadInfo and fetchServerThreadInfos() in the codebase to double check for similar issues.

I didn't find any, but noticed that useKeyserverAdmin accepted ServerThreadInfo which seems strange. Looks like it was added when useKeyserverAdmin was getKeyserverAdmin. Regardless, it's no longer necessary so I removed it.

Depends on D10107

Test Plan: flow + seeing that the hook is only ever used by the CommunityPill react component which should never encounter a ServerThreadInfo.

Reviewers: ginsu, tomek

Reviewed By: ginsu

Differential Revision: https://phab.comm.dev/D10971