remove argName nBlocks from estimatefee RPC command registration

Summary:
Our version of estimatefee does not take a number of blocks as an
argument. It just returns a constant value, and the transaction is
always expected to be in the next block.

Test Plan:
ninja all check-all

$ ./src/bitcoin-cli estimatefee 6
error code: -1
error message:
estimatefee

Estimates the approximate fee per kilobyte needed for a transaction

Result:
n    (numeric) estimated fee-per-kilobyte

Examples:
> bitcoin-cli estimatefee

../test/lint/check-rpc-mappings.py ..

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10075