ELF: Remove dead class that does nothing in class hierarchy.

DefaultTargetHandler is the base class of all <Arch>TargetHandler classes,
and it's the only derived class of TargetHandler class.

TargetHandler and DefaultTargetHandler are actually the same. They define
the same set of pure virtual functions. DefaultTargetHandler is a useless
class in the class hierarchy -- it shouldn't have been added in the first place.

This patch makes all <Arch>TargetHandler classes directly derive from
TargetHandler and removes DefaultTargetHandler.

llvm-svn: 233772