[net] Remove CConnMan::SetAsmap()

Summary:

CAddrMan::m_asmap is now set directly in AppInitMain() so
CConnMan::SetAsmap() is no longer required.

Partial backport of core#22791:
https://github.com/bitcoin/bitcoin/pull/22791/commits/593247872decd6d483a76e96d79433247226ad14

Depends on D12291.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12292