debugger: Remove fake adapter and un-gate GDB (#27557)

This is a clean-up PR in anticipation of introduction of Debugger
Registry. I wanna get rid of DebugAdapterKind (or rather, it being an
enum).
Release Notes:

- N/A

---------

Co-authored-by: Anthony Eid <hello@anthonyeid.me>
Co-authored-by: Anthony <anthony@zed.dev>

crates/assistant_eval/Cargo.toml

@@ -21,6 +21,7 @@ clap.workspace = true
 client.workspace = true
 collections.workspace = true
 context_server.workspace = true
+dap.workspace = true
 env_logger.workspace = true
 fs.workspace = true
 futures.workspace = true