Remove WEBRTC_EXTERNAL_JSON.

The flexibility offered by the GN `rtc_jsoncpp_root` should be enough
to wire a different version of jsoncpp.

Bug: b/281848049
Change-Id: I8af39fec2406e27c3af7b7ec1949a4762dce762f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/304861
Reviewed-by: Jeremy Leconte <jleconte@google.com>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40045}

rtc_base/BUILD.gn

@@ -831,10 +831,6 @@ rtc_library("rtc_json") {
     deps += [ "//third_party/jsoncpp" ]
   } else {
     include_dirs = [ "$rtc_jsoncpp_root" ]
-
-    # When defined changes the include path for json.h to where it is
-    # expected to be when building json outside of the standalone build.
-    defines += [ "WEBRTC_EXTERNAL_JSON" ]
   }
   absl_deps = [ "//third_party/abseil-cpp/absl/strings" ]
 }

rtc_base/strings/json.h

@@ -15,12 +15,7 @@
 #include <vector>
 
 #include "absl/strings/string_view.h"
-
-#if !defined(WEBRTC_EXTERNAL_JSON)
 #include "json/json.h"
-#else
-#include "third_party/jsoncpp/json.h"
-#endif
 
 namespace rtc {