With the demise of ''Walter Cronkite's Universe'' on CBS, the nation's only regularly scheduled science show is ''Nova'' on PBS. However, on Sept. 13 through 17, a new syndicated science show will be appearing mostly on independent stations in the 57 top US markets. It is ''Discover: The World of Science,'' a one-hour special, the first in a series on current developments in science. Science journalists Graham Chedd and John Angier (both past executive producers with ''Nova'') are producing the shows in association with Discover magazine. Well, you gain one, you lose one.