refactor: Replace ParseHashStr with FromHex

No need to have two functions with different names that achieve the
exact same thing.

src/test/fuzz/hex.cpp

@@ -27,8 +27,7 @@ FUZZ_TARGET(hex)
         assert(ToLower(random_hex_string) == hex_data);
     }
     (void)IsHexNumber(random_hex_string);
-    uint256 result;
-    (void)ParseHashStr(random_hex_string, result);
+    (void)uint256::FromHex(random_hex_string);
     (void)uint256S(random_hex_string);
     try {
         (void)HexToPubKey(random_hex_string);