to160 was broken, and we don't use it. So remove it.

src/Conversion.cpp

@@ -42,13 +42,6 @@ bool u160ToHuman(uint160& buf, std::string& retStr)
 
 #endif
 
-base_uint160 uint256::to160() const
-{
- uint160 m;
- memcpy(m.begin(), begin(), m.size());
- return m;
-}
-
 base_uint256 uint160::to256() const
 {
  uint256 m;

src/uint256.h

@@ -511,8 +511,6 @@ public:
 			zero();
 		}
 	}
-
-	base_uint160 to160() const;
 };