Remove `#[cfg(miri)]` from OnceCell tests
Remove `#[cfg(miri)]` from OnceCell tests

They were carried over from once_cell crate, but they are not entirely
correct (as miri now supports more things), and we don't run miri
tests for std, so let's just remove them.

Maybe one day we'll run miri in std, but then we can just re-install
these attributes.

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust