Remove duplicate def of MAX_SLOT_HASHES (#7574)

sdk/benches/slot_hashes.rs

@@ -0,0 +1,23 @@
+#![feature(test)]
+
+extern crate test;
+use solana_sdk::{
+    hash::Hash,
+    slot_hashes::{Slot, SlotHashes, MAX_SLOT_HASHES},
+    sysvar::Sysvar,
+};
+use test::Bencher;
+
+#[bench]
+fn bench_to_from_account(b: &mut Bencher) {
+    let mut slot_hashes = SlotHashes::new(&[]);
+    for i in 0..MAX_SLOT_HASHES {
+        slot_hashes.add(i as Slot, Hash::default());
+    }
+    let mut reps = 0;
+    b.iter(|| {
+        reps += 1;
+        let account = slot_hashes.create_account(0);
+        slot_hashes = SlotHashes::from_account(&account).unwrap();
+    });
+}