Remove the dependency "pycoingecko"

[21isenough]

Actual Behavior:
Currently the ATM relies on the python library "pycoingecko" for the exchange rate. It's a (quick and dirty) replacement for the recently closed API of bitcoinaverage.com

Expected Behavior:
Since this is only one simple GET request to the CoinGecko API, this could simply be done in python and does not need a library. Remove and use the "requests" package in python.

LightningATM/utils.py

Line 53 in 8f81224

 price = CoinGeckoAPI().get_price(ids="bitcoin", vs_currencies=fiat_code)