Remove N extension (user mode interrupts)

[KotorinMinami]

Since the RISC-V N extension draft was removed in the priv spec 1.2 version, the CSR sedeleg is also not listed in the CSR table in the spec. Therefore, it should be removed.

By the way, the implementation of the N extension in the master branch of Sail might be redundant or confusing and lack rigor. I believe it may need to be removed or moved to another branch.