Outdated Arduino cores

[santaimpersonator]

In order to reduce customer confusion and the potential of installing outdated Arduino cores, can we remove the following from package_sparkfun_index.json:

• ESP32 - Now handled by https://github.com/espressif/arduino-esp32
• Apollo3 - As mentioned in #95

To preserve/archive the information, maybe we could create a seperate *.json or branch... in case, uses are relying on the older Arduino cores. (I'm not sure what the best approach would be for these changes.)