[lldb/Scripting] Simplify code by removing the #if.

The "none" script interpreter does not depend on Python so it doesn't
make sense to have it withing the if-block. The only goal seems to be to
have a slightly different error for when there's no script interpreter,
but as per the comment this doesn't make sense for more than one
scripting language. I think the existing error is perfectly clear, so I
just removed this altogether.