[lldb][NFC] Remove dead code that handles situations where LLDB has no dummy target
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.
Reviewed By: labath
Subscribers: abidh, lldb-commits
Differential Revision: https://reviews.llvm.org/D66737