Reverts #1705

This change is safe because:

• the underlying bug in tower::Buffer has been fixed, see Make sure Zebra uses poll_ready and Buffer reservations correctly #1593
• the underlying Inbound service is wrapped in a load_shed layer, so it's not possible for it to hang, even if these requests use up one reservation per clone
• there are 3-4 initial peers by default, and the default buffer size of 20 allows 5+ inbound requests per peer
• some of those inbound requests will immediately fail because the network isn't ready yet, and that helps avoid hangs

See the buffer bound and load shed code:
https://github.com/ZcashFoundation/zebra/blob/main/zebrad/src/commands/start.rs#L71