feat: remove program targeting

[buffalojoec]

Problem

Now that we're stubbing the program account in the transaction context - like the validator runtime does - we can remove the concept of "target programs" in Mollusk.

Summary of Changes

Drop the program_id and program_account fields from the Mollusk struct. Also eliminate now irrelevant API methods.

Now, as long as a program has been added to the cache, Mollusk can be used to target it. Mollusk::new(program_id, program_name) will still create a new instance with the provided program cached.