Remove unstable `Poll::ready`

[ibraheemdev]

Proposal

Poll::ready was added as an experimental alternative to task::ready!, but was deemed to problematic because of the already existing Try implementations for Poll (see rust-lang/rust#89780 for the relevant discussion). Fixing this would likely require changes over an edition, which seems unlikely after task::ready! was already stabilized, so I think we should just go ahead and remove Poll::ready.

Pull request: rust-lang/rust#107060

[m-ou-se]

As discussed in the libs meetup: we can proceed with rust-lang/rust#107060 to remove the unstable .ready() method, as ready!() is already stable. Thanks!