[backport#14818] Bugfix: test/functional/rpc_psbt: Remove check for specific error message that depends on uncertain assumptions

Summary:
c87fc71f7e Bugfix: test/functional/rpc_psbt: Correct test description comment (Luke Dashjr)
097c4aa379 Bugfix: test/functional/rpc_psbt: Remove check for specific error message that depends on uncertain assumptions (Luke Dashjr)

Pull request description:

When converttopsbt is called with a signed transaction, it either fails with "TX decode failed" if one or more inputs were segwit, or "Inputs must not have scriptSigs and scriptWitnesses" otherwise.
Since no effort is made by the test to ensure the inputs are segwit or not, avoid checking the exact message used.
The error code is still checked to ensure it is of the correct kind of failure.

Backport of Core PR14818

Test Plan:

ninja
test_runner rpc_psbt

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6651