[backport#15713 4/5] Remove unused submitToMemoryPool and relayTransactions Chain interfaces

Summary:
https://github.com/bitcoin/bitcoin/pull/15713/commits/b8eecf8e79dad92ff07b851b1b29c2a66546bbc1

Depends on D6421

Partial backport of Core PR15713

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6422