After legacy/trac#26399, none of our crates will have any features. However, we'd like to feature-gate testing C code from Rust (legacy/trac#26398) as a temporary workaround to legacy/trac#25386, so we need to remove the `--all-features` flag. The `--all-features` flags was probably a bad idea to begin with, as many crates have mutually incompatible features (e.g. `curve25519-dalek` has features for different field implementation backends).