Update Rust toolchain to 1.70.0

1.70.0 switched from the git index protocol to the sparse index protocol
as the default. This makes fetches to crates.io much quicker (seconds
instead of potentially minutes).

Remove the unused, non-TOML toolchain file.

Signed-off-by: Tim Crawford <[email protected]>

Author: crawfxrd

rust-toolchain

@@ -1,2 +1,2 @@
 [toolchain]
-channel = "1.61.0"
+channel = "1.70.0"