Last Chance for Famous Froglegs Phil Smidt's Closing on Saturday   [HAMMOND] Phil Smidt's, the famous Hammond restaurant that has served lake perch, frog legs and gooseberry pie to hungry diners since 1910, will lock the doors at 1205 Calumet Avenue for the last time at 9pm on Saturday, October 20. Until then, they will serve lunch and dinner as usual.   Company president, Dave Welch, expressed regret. "Changing demographics are a small part of the story," he said. "We had discussions with the cityi regarding relocating to a smaller, more visible location within Hammond. But ultimately, that didn't happen." Welch isn't ruling out the possiblity of a Phil Smidts reopening elsewhere in Chicagoland or Northwest Indiana.   Phil Smidt's was featured in USA Today's "The 50 Great Places of America" in May 2000 and has long been considered a destination restaurant. Phil Smidt's draws patrons from nearby Indiana and Chicago suburbs and far beyond. Mail orders are not uncommon. "We just shipped out eight frog leg dinners to a family that lives in Nevada," Welch said.   Welch apologizes for any inconvenience to patrons. "Phil Smidts has been a gathering place for wedding receptions, anniversaries, showers, proms and funeral luncheons for 97 years. On behalf of my staff, we thank everyone who has dined here."