[backport#16380] Remove unused bits from the service flags enum

Summary:
fa0d0ff6e1bee60fde63724ae28a51aac5a94d4a Remove unused bits from the service flags enum (MarcoFalke)

Pull request description:

Remove all bits that have no BIP specification nor can be observed on the active network

Backport of Core PR16380

I'm putting this up on the assumption that xthin was a BU only feature
that we can safely remove now that we don't support BCHN-chain anymore.

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8921