do not inspect code for submission completeness when executing script

Fixes #802

If the engine has a `FileName` on it, we are not in the REPL so no submission completeness inspection should happen.
@filipw Glad to see you noticed that these lines were unnecessary ... seem to remember that is caused the engine to "re-use" a session rather then creating one.  We need to remove these from the mono engine as well at some point or when we combine the engine test for Roslyn and Mono.