[lldb-mi] Simplify CMICmnLLDBDebugSessionInfo::Shutdown.

Summary:
* SharedDataDestroy couldn't fail, so no need to return a status.
* No need for status, so can remove error message. The error message
  wasn't displayed or used anywhere anyway.

Reviewers: abidh, ki.stfu

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12879

llvm-svn: 247673