IN what has been termed a landmark case, the Supreme Court ruled in Hills vs. Gautreaux that when the federal government has been guilty of racially discriminatory practices with regard to the siting of city public-housing projects, it can be ordered to adopt a housing assistance plan that ignores municipal boundaries. With this decision, the Court has moved into an area where politicians fear to tread; to many, its action seems imperial and intemperate. Perhaps so- but I would like to suggest that the real problem is that the decision will surely be ineffectual, that the Court cannot act constructively in this area.  To see why, it is important to understand the background of the case.