No longer spin up accounts for client-demo

Now that the Bank is single-threaded again, we can spin up new
accounts on the fly without concern of thread contention. Likewise,
we can send all transactions from a single account, which was also
problematic in the multi-threaded bank. Sending from one account will
also make client-demo straightforward to port to solana-drone.

src/mint.rs

@@ -59,12 +59,6 @@ impl Mint {
     }
 }
 
-#[derive(Serialize, Deserialize, Debug)]
-pub struct MintDemo {
-    pub mint: Mint,
-    pub num_accounts: i64,
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;