[lldb-mi] Remove use of dialog box

Summary:
This really is only implemented on Windows, and it requires us to pull
in User32. This was only useful when debugging on lldb-mi on Windows, and there
doesn't seem to be a good reason why using a dialog box is better than what
exists for other platforms.

Reviewers: zturner, jingham, compnerd

Subscribers: ki.stfu

Differential Revision: https://reviews.llvm.org/D56755

llvm-svn: 351276