openapi3filter: Remove redundant ExcludeResponseBody check

[tatsumack]

This PR removes a redundant condition check in the ValidateResponse function.

Currently, there are two checks for options.ExcludeResponseBody:

// First check at line 91
if options.ExcludeResponseBody {
    // A user turned off validation of a response's body.
    return nil
}

// Second check at line 97 (redundant)
if len(content) == 0 || options.ExcludeResponseBody {
    // An operation does not contains a validation schema for responses with this status code.
    return nil
}

The second check is redundant because if options.ExcludeResponseBody is true, the function would have already returned at the first check. This PR removes the redundant condition to improve code clarity.