refactor: Compute work from headers without CBlockIndex

Avoid the need to construct a CBlockIndex object just to compute work for a header,
when its nBits value suffices for that.

Co-Authored-By: Pieter Wuille <pieter@wuille.net>

src/validation.cpp

@@ -4171,8 +4171,7 @@ arith_uint256 CalculateClaimedHeadersWork(std::span<const CBlockHeader> headers)
 {
     arith_uint256 total_work{0};
     for (const CBlockHeader& header : headers) {
-        CBlockIndex dummy(header);
-        total_work += GetBlockProof(dummy);
+        total_work += GetBlockProof(header);
     }
     return total_work;
 }