remove --all-features from test_rust.sh
After legacy/trac#26399 (moved), none of our crates will have any features. However, we'd like to feature-gate testing C code from Rust (legacy/trac#26398 (moved)) as a temporary workaround to legacy/trac#25386 (moved), 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).