An advisory group established by Lansdale Borough Council has made its recommendation concerning the future of the old Performing Arts Center.   The 311 West Main Street Task Force endorsed a report Tuesday night that calls for a community arts center in the century old building.   Under that scenario, the borough would still own the building but a non-profit group would operate the community arts center.   With its recommendation made, the task force gaveled itself out of existence and put the next step for the old Performing Arts Center in the lap of Borough Council.   It holds its October business meeting next Wednesday.