remove NETWORK_NAME

Summary: This is no longer relevant, now that we support only a single network.

Test Plan:
ninja && src/bitcoind -h | grep version

Check the version number also in src/qt/bitcoin-qt Menu Help -> About Bitcoin ABC

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9107