"Many criminal laws in actual practice do bear with unequal severity upon the poor. It is they who are more likely than the rich to be caught, to be unable to post bail bond, to be prosecuted, to be unskillfully defended, to be convicted and to be punished. However, the remedy for these injustices of society lies in the elimination or mitigation of the conditions and causes of poverty and in the reform of the administration of criminal justice, not by the selective invalidation of otherwise lawfully enacted criminal statutes."