[keyserver] Remove threadTypes.LOCAL from newThreadRequestInputValidator

Summary:
Currently, LOCAL is the same value as COMMUNITY_SECRET_SUBTHREAD, so no changes should occur to validation as a result of this diff.

In a later diff, I'll reassign LOCAL to a new constant. Going forward, LOCAL will be used for thick threads only, which should never be created by a keyserver. So it makes sense to leave this out of newThreadRequestInputValidator.

Depends on D12482

Test Plan: Flow

Reviewers: kamil, marcin

Reviewed By: kamil

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12483