chore: delete MemoryDB (AztecProtocol#4241)

This module is not being used anywhere and was just being kept around as
a reference. However this results in a maintenance burden as this shares
tests with the KV DB, which means we have to keep feature parity (see
e.g. AztecProtocol#4238).

The KV DB is in a good place already, and we can always look at the repo
history if we wish to bring this back, so the simplest path forward is
to remove this.

yarn-project/end-to-end/src/e2e_persistence.test.ts

@@ -153,7 +153,7 @@ describe('Aztec persistence', () => {
 
       expect(ownerBalance).toEqual(initialOwnerBalance - 500n);
       expect(targetBalance).toEqual(500n);
-    });
+    }, 30_000);
   });
 
   describe.each([

yarn-project/pxe/src/database/index.ts

@@ -1,2 +1 @@
 export * from './pxe_database.js';
-export * from './memory_db.js';