Revert "[lldb] Reinstate original guard variable check"

It seems this breaks the following tests:
    lldb-Suite :: expression_command/call-function/TestCallUserDefinedFunction.py
    lldb-Suite :: expression_command/rdar42038760/TestScalarURem.py

Let's revert this patch and wait until we find an actual issue that could be
fixed by also doing the guard variable check on Windows.

llvm-svn: 368920