Remove futures_api feature flag

[marcelbuesing]

This removes all occurrences of the futures_api feature flag.

Motivation and Context

Since https://github.com/rust-lang/rust/pull/59739 has been merged it should no longer be necessary to specify the futures_api feature flag. The minimal nightly version required is rustc 1.36.0-nightly (e305df184 2019-04-24).

Building the runtime crate using this nightly version otherwise shows:

warning: the feature `futures_api` has been stable since 1.36.0 and no longer requires an attribute to enable

FYI: Also see this pull request: https://github.com/rust-lang-nursery/futures-rs/pull/1546/files

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

I think this in a way a breaking change as it changes the error behavior of rust stable and nightly versions that are older than nightly 2019-04-24. Nevertheless since the features async_await and await_macro are not yet stabilized in stable < 1.36 the error that nightly rust is required should still be shown, just not specifically mentioning the futures_api feature.

[yoshuawuyts]

@marcelbuesing I'm debating whether or not to publish a new release for all crates based on this patch, or wait for more patches to land before doing a release. Do you have a preference?