The last vestige of Depression-era rules regulating the cottage industry in apparel is about to be abandoned. At the urging of Vice-President Dan Quayle's Council on Competitiveness, Labor Secretary Lynn Martin is planning to overhaul rules that prohibit the home manufacture of women's garments and of jewelry-making that involves hazardous substances. Needle-trade unions won enactment of the regulations--and have long defended them--on the grounds that officials have no way to make sure that home-based operations follow wage, safety, and child-labor laws. Of course, home workers are also all but impossible to organize. And the end of the ban is one more measure of the waning clout of unions.