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/dap_adapters/Cargo.toml

@@ -30,7 +30,6 @@ paths.workspace = true
 regex.workspace = true
 serde.workspace = true
 serde_json.workspace = true
-sysinfo.workspace = true
 task.workspace = true
 util.workspace = true