remove pin-utils

[yoshuawuyts]

Since #381 we've moved from pin-utils to pin-project-lite. We should finish that migration, and stop relying on pin-utils entirely.

[taiki-e]

pin-project{-lite} doesn't support stack-pinning (pin_mut!), so pin-utils still seems necessary.

[yoshuawuyts]

@taiki-e ah yeah, that makes sense. Thanks for clarifying!