[net processing] Default initialize m_recent_confirmed_transactions

Summary:
Now that m_recent_confirmed_transactions is owned by PeerManagerImpl,
and PeerManagerImpl's lifetime is managed by the node context, we can
just default initialize m_recent_confirmed_transactions during object
initialization. We can also remove the unique_ptr indirection.

This concludes backport of core#21562
https://github.com/bitcoin/bitcoin/pull/21562/commits/fde1bf4f6136638e84cdf9806eedaae08e841bbf

Depends on D13379

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13380