rust: remove out-of-date comment
We've migrated to a newer version of Rust, so it doesn't make sense anymore.
--- a/rust/hg-core/Cargo.toml Mon Mar 06 12:00:25 2023 +0100
+++ b/rust/hg-core/Cargo.toml Mon Mar 06 12:04:25 2023 +0100
@@ -36,8 +36,6 @@
memmap2 = { version = "0.5.8", features = ["stable_deref_trait"] }
zstd = "0.12"
format-bytes = "0.3.0"
-# once_cell 1.15 uses edition 2021, while the heptapod CI
-# uses an old version of Cargo that doesn't support it.
once_cell = "1.16.0"
# We don't use the `miniz-oxide` backend to not change rhg benchmarks and until