[lldb][NFC] Remove dead code that handles situations where LLDB has no dummy target

Summary:
We always have a dummy target, so any error handling regarding a missing dummy target is dead code now.
Also makes the CommandObject methods that return Target& to express this fact in the API.

This patch just for the CommandObject part of LLDB. I'll migrate the rest of LLDB in a follow-up patch that's WIP.

Reviewers: labath

Reviewed By: labath

Subscribers: abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D66737

llvm-svn: 369939