[TieredStorage] Remove the general-purposed TieredStorageWriter (#196)

#### Problem
tiered_storage/writer.rs was added when we planned to support multiple
tiers in the tiered-storage (i.e., at least hot and cold).  However, as we
changed our plan to handle cold accounts as state-compressed accounts,
we don't need a general purposed tiered-storage writer at this moment.


#### Summary of Changes
Remove tiered_storage/writer.rs as we currently don't have plans to develop cold storage.

#### Test Plan
Existing tiered-storage tests.

Author: yhchiang-sol

accounts-db/src/tiered_storage.rs

@@ -11,7 +11,6 @@ pub mod mmap_utils;
 pub mod owners;
 pub mod readable;
 mod test_utils;
-pub mod writer;
 
 use {
     crate::{