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/zed/Cargo.toml

@@ -41,6 +41,8 @@ command_palette.workspace = true
 command_palette_hooks.workspace = true
 component_preview.workspace = true
 copilot.workspace = true
+dap.workspace = true
+dap_adapters.workspace = true
 debugger_ui.workspace = true
 debugger_tools.workspace = true
 db.workspace = true
@@ -149,6 +151,7 @@ ashpd.workspace = true
 
 [dev-dependencies]
 call = { workspace = true, features = ["test-support"] }
+dap = { workspace = true, features = ["test-support"] }
 editor = { workspace = true, features = ["test-support"] }
 gpui = { workspace = true, features = ["test-support"] }
 image_viewer = { workspace = true, features = ["test-support"] }