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/workspace/Cargo.toml

@@ -35,6 +35,7 @@ client.workspace = true
 clock.workspace = true
 collections.workspace = true
 component.workspace = true
+dap.workspace = true
 db.workspace = true
 derive_more.workspace = true
 fs.workspace = true
@@ -68,6 +69,7 @@ zed_actions.workspace = true
 [dev-dependencies]
 call = { workspace = true, features = ["test-support"] }
 client = { workspace = true, features = ["test-support"] }
+dap = { workspace = true, features = ["test-support"] }
 db = { workspace = true, features = ["test-support"] }
 env_logger.workspace = true
 fs = { workspace = true, features = ["test-support"] }