Remove superfluous and broken manifest file (#133)

`library.json` is PlatformIO's native library manifest file but Arduino's `library.properties` is also supported by
PlatformIO:

https://docs.platformio.org/en/latest/librarymanager/creating.html#creating-library

The `library.json` file was missing the required `version` field:

https://docs.platformio.org/en/latest/librarymanager/config.html#version

Although it could be added, experience with other libraries indicates that it is not really realistic to expect two
manifest files to be properly maintained.

Since the Arduino development software and the Arduino Library Manager requires `library.properties`, if we are to have
only one the choice of keeping `library.properties` and removing `library.json` is clear.

Author: per1234