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/debugger_ui/Cargo.toml

@@ -8,6 +8,10 @@ license = "GPL-3.0-or-later"
 [lints]
 workspace = true
 
+[lib]
+path = "src/debugger_ui.rs"
+doctest = false
+
 [features]
 test-support = [
     "dap/test-support",