We've had two ways to print a "debug" log message.
- Log::GetDebug() was testing a Stream flag which was never set.
- Log::Debug() was checking for the presence of "log enable --debug"
flag.
Given that these two were used very rarely and we already have a
different way to specify "I want a more verbose log", I propose to remove
these two functions and migrate the callers to LLDB_LOGV. This commit
does that.