Run CI with default cargo features (incl. nvidia), drop ci, `benc…

…hmark` features (#2782)

* Remove the `benchmarks` feature flag

We've removed the last usage of it back in 2020 in a9514e0268ec805c6d47.

Now _default features_ == _all features_, so there's no longer need to pass `--all-features` if you want to compile everything. Only exception is the `offline` feature of `visual-config`, but that actually removes functionality rather than adding it.

* Run CI with default cargo features, drop `ci` feature

The only addition in `default` on top of `ci` was the `nvidia` feature, and it turns out CI passes just fine with it enabled.

Also clean up the `[features]` sections of various `Cargo.toml`s.

Cargo.toml

@@ -13,6 +13,3 @@ tonari-math = { path = "../tonari-math" }
 
 [build-dependencies]
 cxx-build = "1.0"
-
-[features]
-default = []