Remove `bugreport` command

The bugreport command exists to create domain-specific bug reports.
Currently it has one implementation for filing bugs on the unwinder. As
far as we can tell, it has never been of use. Although not exactly the
same as the reproducers, it's a bit confusing to have two parallel
command trees for (kind of) the same thing.

Differential revision: https://reviews.llvm.org/D65469

llvm-svn: 371132