[build] Move rapidjson back to third_party

Previously rapidjson were moved to zircon/third_party in
order to prevent abs paths to appear in Fuchsia build. This is no
longer needed and should be reverted back.

Bug: 57392
Change-Id: I4f65524693a428e14c9bbc85450f3327ca4eb1e5
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/github.com/google/cppdap/+/476224
Reviewed-by: Petr Hosek <phosek@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index aa56c21..1f9f24f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -33,10 +33,7 @@
 
   public_configs = [ ":includes" ]
 
-  deps = [
-    # TODO(fxbug.dev/57392): Move it back to //third_party once unification completes.
-    "//zircon/third_party/rapidjson",
-  ]
+  deps = [ "//third_party/rapidjson" ]
 }
 
 if (is_host) {