Incorrect comment hints at security problem with channel key reuse

`hsmd.c` has the following comment:

```c++
/*~ This is a unique value handed to us from lightningd, used for
* per-channel seed generation (a single id may have multiple channels
* over time).
*
* It's actually zero for the initial lightningd client connection and
* the ones for gossipd and connectd, which don't have channels
* associated. */
u64 dbid;
```

This is hopefully not correct, because reusing channel keys is a security problem (revocation secrets must not be reused).