Deprecate unused `FileID` fields in contract operations, fix documentation

[tinker-michaelj]

Description

The init code used to create a contract is in the file referenced by the FileID in the ContractCreateTransactionBody.

However, the resulting bytecode is immutable, and not stored at the original FileID (or any FileID).

So we should deprecate the FileID in the ContractGetInfo and ContractUpdateTransactionBody.