Inline payment_plan

This module predates Accounts. That was a separate module because
it used to be part of Bank and those types could be sent to any
smart contract. Now each instruction processor defines for itself
what instructions it accepts.

programs/budget_api/src/lib.rs

@@ -2,7 +2,6 @@ pub mod budget_expr;
 pub mod budget_instruction;
 pub mod budget_processor;
 pub mod budget_state;
-pub mod payment_plan;
 
 use solana_sdk::pubkey::Pubkey;