[lldb-mi] Remove unused bool results.

Summary:
Many methods, in particular various 'Add' methods didn't have
any actual failure scenarios that were being emitted. This meant
that a lot of surrounding code could be simplified.

Reviewers: abidh, ki.stfu

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11412

llvm-svn: 242911