runtime: remove sizespecializedmalloc GOEXPERIMENT in Go 1.28

[matloob]

We plan to ship sizespecializedmalloc on in Go 1.27 with the ability to turn it off, with the plan to remove it in Go 1.28. This issue tracks the removal of the experiment in Go 1.28.

[gabyhelp]

Related Issues

• runtime: sizespecializedmalloc experiment #79286 (closed)

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