Merge bitcoin/bitcoin#27325: test: various `converttopsbt` check cleanups in rpc_psbt.py

afc2dd54848fa70ed408ae259420ff8648f21efc test: various `converttopsbt` check cleanups in rpc_psbt.py (Sebastian Falbesoner)

Pull request description:

  In the functional test rpc_psbt.py, some comments around the `converttopsbt` RPC checks are wrong or outdated and can be removed:

  > _Error could be either "TX decode failed" (segwit inputs causes
  > parsing to fail) or "Inputs must not have scriptSigs and
  > scriptWitnesses"_

  Decoding a valid TX with at least one input always succeeds with the [heuristic](https://github.com/bitcoin/bitcoin/blob/e352f5ab6b60ec1cc549997275e945238508cdee/src/core_read.cpp#L126), i.e. this comment is not right and we can assert for the error string "Inputs must not have scriptSigs and scriptWitnesses" on the calls below.

  > _We must set iswitness=True because the serialized transaction has
  > inputs and is therefore a witness transaction_

  This is also unneeded (and confusing, w.r.t. "is therefore a witness transaction"?), for a TX with one input there is no need to set the `iswitness` parameter. For sake of completeness, we still keep one variant where iswitness is explicitly set to true.

  Lastly, there is a superflous `converttopsbt` call on the raw tx which is the same as just [about ~10 lines above](https://github.com/bitcoin/bitcoin/blob/master/test/functional/rpc_psbt.py#L393-L397), so it can be removed.

ACKs for top commit:
  ismaelsadeeq:
    tested ACK afc2dd54848fa70ed408ae259420ff8648f21efc
  achow101:
    ACK afc2dd54848fa70ed408ae259420ff8648f21efc

Tree-SHA512: 467efefdb3f61efdb79145044b90fc8dc2f0c425f078117a99112b0074e7d4a32c34e464f665fbf8de70d06caaa5d4ad5908c1d75d2e7607eccb0837480afab3

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