[backport#15245] remove deprecated mentions of signrawtransaction from fundraw help

Summary:
851380ce17 remove deprecated mentions of signrawtransaction from fundraw help (Gregory Sanders)

Pull request description:

RPC call has been removed as of 0.17.99.

Backport of Core PR15245

Test Plan:

none

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6846