Removed unnecessary call to OpenFirstAvailableMaster.

::Fork already does this internally, so this was simply leaking file handles.

This fixes the problem where the test suite would occasionally run out of file handles.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@192929 91177308-0d34-0410-b5e6-96231b3b80d8