[HCB] Remove `api/fiscal-sponsorship/*` endpoints

[garyhtou]

The endpoints under api/fiscal-sponsorship/* are no longer used because we've moved the application into the HCB codebase. These can be removed. Make sure also to remove any associated code (such as usages of these endpoints — although I'd assume those are already gone/dead/commented-out).

Anyone can take on this ticket — you don't need to be on the HCB team! Once you have a PR, link this ticket in the PR and assign @/YodaLightsabr for review.

[sadeshmukh]

on it!