Check !flags.isDebug directly in Trace.isActivelyTracing.

Problem

Non-debug TraceIds with _sampled=Some(false) sent over the wire have the
SamplingKnown flag set. Trace.isActivelyTracing would match Some(false)
but only if flags is zero, but here it is two.

Solution

The case clause to find non-sampled, non-debug TraceIds was changed to
directly check flags.isDebug instead of restricting flags to zero.