[net processing] Default initialize recentRejects

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

This is a partial backport of core#21562
https://github.com/bitcoin/bitcoin/pull/21562/commits/cd9902ac5054c01228d52616bf85f7196364d4ff

Depends on D13377

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13378