[stdlib] remove unnecessary mod.rs file
[stdlib] remove unnecessary mod.rs file

Not sure why this file exists. There's no comment explaining why and the build still passes so deleting it is appropriate.

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