Remove ui test for doc_cfg feature gate

Author: GuillaumeGomez

tests/ui/feature-gates/feature-gate-doc_cfg.rs

@@ -1,2 +1,6 @@
-#[doc(cfg(unix))] //~ ERROR: `#[doc(cfg)]` is experimental
+//@ build-pass
+
+// FIXME: Remove this test once `doc_cfg` feature is completely removed.
+
+#[doc(cfg(unix))]
 fn main() {}