Remove redundant and outdated library metadata file

library.json is PlatformIO's native library manifest file but library.properties is also supported:
https://docs.platformio.org/en/latest/librarymanager/creating.html

The library.json file has not been maintained. The version is outdated and it also has an incorrect `platforms` value. So the evidence makes it obvious that having two metadata files in this project has a harmful effect. Since the Arduino development software requires library.properties, the choice of keeping library.properties and removing library.json is clear.

library.json

@@ -1,24 +0,0 @@
-{
-  "name": "NTPClient",
-  "keywords": "ntp, client, time",
-  "description": "A NTPClient to connect to a time server",
-  "authors":
-  [
-    {
-      "name": "Fabrice Weinberg",
-      "email": "fabrice@weinberg.me"
-    },
-    {
-      "name": "Sandeep Mistry",
-      "email": "s.mistry@arduino.cc"
-    }    
-  ],
-  "repository":
-  {
-    "type": "git",
-    "url": "https://github.com/arduino-libraries/NTPClient.git"
-  },
-  "version": "3.1.0",
-  "frameworks": "arduino",
-  "platforms": "espressif"
-}