refactor: Remove nMyStartingHeight from CNode/Connman
Summary:
CNode and CConnman keep track of the active chain height when CNodes have been created, but apart from serializing the int once (when sending a version message), it is unused. So it can simply be removed in favor of a single int in PeerMan that can do the same.
Backport of core#20649.
Depends on D10884.
Ref T1696.
Test Plan:
ninja all check-all ninja bitcoin-fuzzers
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Subscribers: PiRK
Maniphest Tasks: T1696
Differential Revision: https://reviews.bitcoinabc.org/D10885