rpc: Remove deprecated migration code

Summary:
Don't accept a second argument to sendrawtransaction and
testmempoolaccept of type bool. Actually even the code before this
change would not accept bool, but it would print a long explanatory
message when rejecting it: "Second argument must be numeric (maxfeerate)
and no longer supports a boolean. To allow a transaction with high fees,
set maxfeerate to 0."

This was scheduled for removal in rABC1d4e5281a755

This is a backport of Core PR18495

Test Plan:
ninja all check-all

src/bitcoin-cli help testmempoolaccept
src/bitcoin-cli help sendrawtransaction

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8919