[lldb-mi] Remove a test that uses pexpect().

Summary:
Its functionality is entirely covered by exec-run.test (which
doesn't use pexpect)

Reviewers: serge-sans-paille

Subscribers: ki.stfu, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58459

llvm-svn: 354494