Remove old/unmaintained code in the dbug/ directory.
Moved dbug.cc to mysys, remove dbug library.
Remove unused functions in dbug.cc
Remove DBUG_LEAVE macro, no longer needed.
Wrap body of DBUG_LOG macro in if _db_enabled_(), to avoid formatting
of strings that likely will not be printed anyways.

Change-Id: I15bf081bebbc41886f5bed29ad21eae35dd6f387