[keyserver] Delete setup.sh

Summary: Per @ashoat's comment on D4714. Relevant Linear issue here.

Test Plan: Ran git grep setup.sh and found no occurrences of setup.sh in the codebase.

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: tomek, adrian, ashoat

Differential Revision: https://phab.comm.dev/D4726