Clean up debug logging

Summary:
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.

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D29823

llvm-svn: 294939