Change features test to run on the main toolchain

[robertbastian]

Clippy now detects methods that aren't in the MSRV: https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv

We could change the all-features job to use the main toolchain, which would be convenient for local development, as using the MSRV toolchain requires a full rebuild.

[robertbastian]

Oh and even better, this respects per-crate MSRV, so we can get rid of the msrv-utils job