obsexchange: delete dead code for old exchange protocol
Dead since 79a926b557f1 (compat: drop support for older exchange protocol, 2017-10-25).
All legacy (hg-git based) Projects have been automatically migrated to native with the upgrade of 2025-01-23
Dead since 79a926b557f1 (compat: drop support for older exchange protocol, 2017-10-25).