Deprecate nblocks from estimatefee RPC

Summary:
nblocks is no longer necessary, as the implementation does not use it.
Depends on D2614
Split out from D2543 for better review

Test Plan:

ninja check
make check
test_runner.py rpc_estimatefee

bitcoind
bitcoin-cli estimatefee # succeeds
bitcoin-cli estimatefee 2 # errors as expected

bitcoind -deprecatedrpc=estimatefee
bitcoin-cli estimatefee # succeeds
bitcoin-cli estimatefee 2 # succeeds

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2615