Drop the BITCOIN_CASH service bit

Summary: This is no longer needed.

Test Plan:

ninja all check-all

Check with:

git grep NODE_BITCOIN_CASH

That remaining occurrences are deliberate or documentation archives.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8524