[rust] remove 'set_stdio' nightly feature
[rust] remove 'set_stdio' nightly feature

Now that there are no more uses of the `set_stdio` nightly feature
remove it from the list of whitelisted nightly features.

Closes: #284

aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core