Greta Garbo is alone at last. The sultry MGM star of the ’30s — whose signature line was “I vant to be alone” — died in New York City in 1990 at the age of 84. She was cremated, and her ashes were kept at a New York mortuary while her niece and sole heir, Gray Reisfield, decided what to do with them. Finally, yesterday, action was taken in Stockholm, Sweden, the star’s native city. To avoid the media and curious onlookers, Garbo’s relatives tricked people into thinking the ashes would be buried Thursday at Woodland Cemetery instead of Wednesday. Garbo’s small urn was lowered into the grave by blue and gold ribbons, the colors of Sweden’s flag. Only a handful of family and friends were present.