util: remove unused IsHexNumber

The relevant unit tests have been incorporated in
uint256_tests/from_user_hex in a previous commit.

src/test/fuzz/hex.cpp

@@ -27,7 +27,6 @@ FUZZ_TARGET(hex)
     if (IsHex(random_hex_string)) {
         assert(ToLower(random_hex_string) == hex_data);
     }
-    (void)IsHexNumber(random_hex_string);
     if (uint256::FromHex(random_hex_string)) {
         assert(random_hex_string.length() == 64);
         assert(Txid::FromHex(random_hex_string));