PR13396 backport https://github.com/bitcoin/bitcoin/pull/13396/files
2acd1d6716959b99e751cf85a7c47aaa383e937f Drop uint 256 not operator (Ben Woosley)
Pull request description:
All the other operators are integer or bitwise operations, and this is unused
apart from tests.
Note attempting to call `!` on `arith_uint256` results in a build error after this change:
```
test/arith_uint256_tests.cpp:201:17: error: invalid argument type 'const arith_uint256' to unary expression
BOOST_CHECK(!ZeroL);
```