[RPC] Remove deprecated fee-related mempool entry fields

Summary:
They have all been deprecated for very long and replaced by the fees object field that provides the same data with better encpsulation.

Depends on D13135.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13136