[llvm] CallSiteSplitting: Replace -callsite-splitting-duplication-threshold cl::opt with pass parameter (PR #223479)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Instead of converting, do we just want to remove? Is anyone using this option?

Same as the others, this is a pass testing option someone might want to experiment with. But it's not worth the cost of a cl::opt, but pass arguments are cheap 

https://github.com/llvm/llvm-project/pull/223479