The meeting of the Towamencin Zoning Hearing Board that was scheduled for Thursday evening has been canceled.   The agenda included zoning variance requests for an 89-room hotel proposed at Bustard Road and Franklin Street in Kulpsville but township officials say the developer withdrew its application.   That action leaves in limbo a conditional use hearing for the project the Towamencin supervisors opened but continued until an unspecified future date.   The zoning variances sought by the developer included relief from setback and building height requirements.