Explicitly unset PYTHONPATH in wrapper script

MP calls the wrapper from a shell in the CXCDS environment with an
incompatible PYTHONPATH.
Unset LD_LIBRARY_PATH and SYBASE

These are already being unset in the MP line that calls starcheck:

"/usr/bin/env LD_LIBRARY_PATH='' SYBASE=''/proj/sot/ska/bin/starcheck.pl ";

but now that we're using a wrapper, we might as well explicitly control
everything we need to explicitly control.
Merge pull request #68 from sot/tweak_wrapper

Tweak wrapper for integration with SOT MP script