Merge #12942: rpc: Drop redundant testing of signrawtransaction prevtxs args

Summary:
459ea58 rpc: Drop redundant testing of signrawtransaction prevtxs args (Ben Woosley)

Pull request description:

These other types are already tested on line 736.

Tree-SHA512: 2efe777c8a63c69ffe0fafcb2f37f134d324a8bc9525510f1079d2215535b511d6308e5e6eec702a3444f87701236c5e7a22f10bb24e5a454010ef421e5ae900

Backport of Core PR12942
https://github.com/bitcoin/bitcoin/pull/12942/

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3842