[DBSP] Remove the `add_input_set` API

Suggested by @ryzhyk in https://github.com/feldera/feldera/pull/6755/files/22b1d358059db7d3b726839bb05718f0824e10ed#diff-448b36f17ad37007adaa27b8b49710d7e51796c447e27920aa834c38a2c6f44f