[avalanche] Add a removetransaction RPC to force remove a mempool transaction

Summary: This can be used to remove a transaction from the mempool/block template, for example to make room for another transaction instead when coupled with the finalizetransaction RPC.

Test Plan:

./test/functional/test_runner.py abc_rpc_finalizetransaction

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18793