Remove picojson submodule

We use vjson now

Author: zpostfacto

.gitmodules

@@ -5,9 +5,6 @@
 [submodule "src/external/webrtc"]
 	path = src/external/webrtc
 	url = https://webrtc.googlesource.com/src
-[submodule "src/external/picojson"]
-	path = src/external/picojson
-	url = https://github.com/kazuho/picojson.git
 [submodule "src/external/vjson"]
 	path = src/external/vjson
 	url = https://github.com/zpostfacto/vjson.git