Trim down xtensa crates

[bugadani]

This PR removes some legacy code from the xtensa crates.

[MabezDev]

esp-rs/esp-pacs#282 has been merged, I think we can get this ready now.

[bugadani]

What should we do about esp_hal::sync? cortex-m and riscv define their own critical sections, so if we can somehow configure a core count for xtensa-lx we might do the same and move the impl there.

Or should we just provide a single-core feature and implemet a single-core critical section in xtensa-lx?

I'm not entirely sure how far we want to generalize this crate at this moment. I'm also not sure whether that question is in scope of this PR :)