Remove the old SplitChunksPlugin from the codebase

[hyf0]

After implementing the new SplitChunksPlugin and the tests being passed, Rspack enable the new SplitChunksPlugin by default. It brings better performance and stability.And there's one issue remains unresolved, which is #3120.

But now #3120 is solved, which leave us no reasons to keep the old SplitChunksPlugin anymore.

So I think it's time to remove the old SplitChunksPlugin from the codebase, it will reduce the binary size of rspack and complexity of the codebase.

I don't consider this a breaking change, since the new SplitChunksPlugin is enable by default and we are removing a option in experiments.

[hardfist]

it would be better it's released in 0.4.0 incase different behavior caused by new Implementation and old Implementation inconsistencies, people may rely on some css bug caused by old behavior

[hyf0]

Agree. I will do the remove step at #4041.