Don't pull in entire JSON repo for single header file

.gitmodules

@@ -31,6 +31,3 @@
 [submodule "cpr"]
     path = externals/cpr
     url = https://github.com/whoshuu/cpr.git
-[submodule "json"]
-    path = externals/json
-    url = https://github.com/nlohmann/json.git

externals/CMakeLists.txt

@@ -62,5 +62,5 @@ if (ENABLE_WEB_SERVICE)
 
     # JSON
     add_library(json-headers INTERFACE)
-    target_include_directories(json-headers INTERFACE ./json/src)
+    target_include_directories(json-headers INTERFACE ./json)
 endif()