unused function 'insertSeenValue'

[nsmithtt]

When compiling with -Werror:

shardy/dialect/sdy/transforms/propagation/debugging/source_sharding.cc:349:6: error: unused function 'insertSeenValue' [-Werror,-Wunused-function]
  349 | bool insertSeenValue(Operation* op, const PropagationEdge& edge,

[ZixuanJiang]

Thank you. We remove it in #947.