Language Accepted by a Turing Machine with a Useless State?
The definition of the language accepted by a Turing machine is valid for all Turing machines, whether they have useless states or not. Absolutely nothing changes. If there is a useless state, removing it wouldn't change the language accepted by the machine, as does adding another useless state.

Consider a similar question about C programs. What if I have a function which is never called? Does this affect the way the program executes? It doesn't.1 The same holds for Turing machines.

1Actually it might, since you might run out of memory a tiny bit faster.