descriptorprocesspsbt RPC uses a different flow internally from the walletprocesspsbt RPC, which was already tested in the tests.
This patch ensures that descriptorprocesspsbt RPC can correctly sign the PSBT while removing the non witness UTXOs in case of Segwit V1 only inputs.
This was identified in the review of #31622 PR. Ref: #31622 (review)
<!-- *** Please remove the following help text before submitting: *** Pull requests without a rationale and clear improvement may be closed immediately. GUI-related pull requests should be opened against https://github.com/bitcoin-core/gui first. See CONTRIBUTING.md -->
<!-- Please provide clear motivation for your patch and explain how it improves Bitcoin Core user experience or Bitcoin Core developer experience significantly: * Any test improvements or new tests that improve coverage are always welcome. * All other changes should have accompanying unit tests (see `src/test/`) or functional tests (see `test/`). Contributors should note which tests cover modified code. If no tests exist for a region of modified code, new tests should accompany the change. * Bug fixes are most welcome when they come with steps to reproduce or an explanation of the potential issue as well as reasoning for the way the bug was fixed. * Features are welcome, but might be rejected due to design or scope issues. If a feature is based on a lot of dependencies, contributors should first consider building the system outside of Bitcoin Core, if possible. * Refactoring changes are only accepted if they are required for a feature or bug fix or otherwise improve developer experience significantly. For example, most "code style" refactoring changes require a thorough explanation why they are useful, what downsides they have and why they *significantly* improve developer experience or avoid serious programming bugs. Note that code style is often a subjective matter. Unless they are explicitly mentioned to be preferred in the [developer notes](/doc/developer-notes.md), stylistic code changes are usually rejected. -->
<!-- Bitcoin Core has a thorough review process and even the most trivial change needs to pass a lot of eyes and requires non-zero or even substantial time effort to review. There is a huge lack of active reviewers on the project, so patches often sit for a long time. -->
