including AdafruitIO_FONA.h causes build issues

[BioSLuDge]

• Arduino board: Adafruit FONA feather

• Arduino IDE version: 1.8.13

• List the steps to reproduce the problem below (if possible attach a sketch or
  copy the sketch code in too): Just try to build any Adafruit IO sketch with AdafruitIO_FONA.h, it will fail to build because the FONA library doesn't implement the _disconnect method which is required to fully implemented by the base AdafruitIO class.

I added void _disconnect() {}; to the protected section of the FONA library and it builds and runs. But I'm not sure if more should be in there for proper long term function or not.

[brentru]

@BioSLuDge Hi, added a draft PR which fixes this build error and disables the GPRS module.

#135