[validation] Always call mempool.check() after processing a new transaction

Summary:
CTxMemPool::check() will carry out internal consistency checks 1/n times,
where n is set by the -checkmempool configuration option. By default,
mempool consistency checks are disabled entirely on mainnet.

Therefore, this change has no effect on mainnet nodes running with
default configuration. It simply removes the responsibility to trigger
mempool consistency checks from net_processing.

This concludes a backport of core#23173
https://github.com/bitcoin/bitcoin/pull/23173/commits/0fdb619aaf1d62598263361a6082d182be1af792

Depends on D12235

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12236