partial revert of tx decode sanity check backport

Summary:
(partial revert of D4247)

This Diff had the unfortunate side effect of rendering real
on-chain transactions to be un-decodeable in decoderawtransaction. As
seen in the original PR, the rationale for this loss of functionality was
that the segwit transaction format is ambiguous with 0-input transactions
used during transaction construction.

In the case of BCH, we don't need to damage our RPC for the sake of segwit.

Test Plan:

make check
test_runner.py

• try decoding mainnet tx ebc9fa1196a59e192352d76c0f6e73167046b9d37b8302b6bb6968dfd279b767 and ensure it does not fail (has truncated scripts):
  
  bitcoin-cli decoderawtransaction 01000000019ac03d5ae6a875d970128ef9086cef276a1919684a6988023cc7254691d97e6d010000006b4830450221009d41dc793ba24e65f571473d40b299b6459087cea1509f0d381740b1ac863cb6022039c425906fcaf51b2b84d8092569fb3213de43abaff2180e2a799d4fcb4dd0aa012102d5ede09a8ae667d0f855ef90325e27f6ce35bbe60a1e6e87af7f5b3c652140fdffffffff080100000000000000010101000000000000000202010100000000000000014c0100000000000000034c02010100000000000000014d0100000000000000044dffff010100000000000000014e0100000000000000064effffffff0100000000

• also try decoding mainnet tx f6e637d3469a552174b80bd10972d137a4ce4be519910e455bde91339514dc18 (very long scriptpubkey, too long to paste here):

bitcoin-cli decoderawtransaction `bitcoin-cli getrawtransaction f6e637d3469a552174b80bd10972d137a4ce4be519910e455bde91339514dc18`
`

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4665