Remove the -deprecatedrpc=softforks option

Summary:
This has been deprecated since v0.27.0 and was returning an empty object anyway, so it's safe to remove.

Depends on D14370.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14371