init: avoid unsetting service bits from nLocalServices

Summary:
Rather than setting the service bit NODE_NETWORK first and then unset
it, start out the bare minimum flags that every node serves and only add
NODE_NETWORK if we are running as a non-pruned node.

This is a backport of core#25887

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17962