delete LfsFallbackRemoteStore

Summary:
Grepping shows it's not longer used anywhere. It's safe to delete altogether. This will also fix 2 warnings:

```
warning: associated function `new` is never used
    --> fbcode/eden/scm/lib/revisionstore/src/lfs.rs:1993:12
     |
1992 | impl LfsFallbackRemoteStore {
     | --------------------------- associated function in this implementation
1993 |     pub fn new(wrapped_store: Arc<dyn RemoteDataStore>) -> Arc<dyn RemoteDataStore> {
     |            ^^^

warning: associated function `new` is never used
    --> fbcode/eden/scm/lib/revisionstore/src/lfs.rs:1993:12
     |
1992 | impl LfsFallbackRemoteStore {
     | --------------------------- associated function in this implementation
1993 |     pub fn new(wrapped_store: Arc<dyn RemoteDataStore>) -> Arc<dyn RemoteDataStore> {
     |            ^^^
     |
     = note: `#[warn(dead_code)]` on by default

```

Reviewed By: quark-zju

Differential Revision: D65218832

fbshipit-source-id: 55825e13e9e1d3367f2da58c15e86768462b41e9

Author: MichaelCuevas

eden/scm/lib/revisionstore/src/lfs.rs

@@ -1980,78 +1980,6 @@ impl LocalStore for LfsRemoteStore {
     }
 }
 
-/// Wraps another remote store to retry fetching content-keys from their HgId keys.
-///
-/// If for any reason, the LFS server is turned off, we will end up in here for blobs where we have
-/// the pointer locally, but not the blob.  In this case, we want the code to fallback to fetching
-/// the blob with the regular non-LFS protocol, hence this stores merely translates
-/// `StoreKey::Content` onto `StoreKey::HgId` before asking the non-LFS remote store to fetch data
-/// for these.
-pub struct LfsFallbackRemoteStore(Arc<dyn RemoteDataStore>);
-
-impl LfsFallbackRemoteStore {
-    pub fn new(wrapped_store: Arc<dyn RemoteDataStore>) -> Arc<dyn RemoteDataStore> {
-        Arc::new(Self(wrapped_store))
-    }
-}
-
-impl RemoteDataStore for LfsFallbackRemoteStore {
-    fn prefetch(&self, keys: &[StoreKey]) -> Result<Vec<StoreKey>> {
-        let mut not_found = Vec::new();
-        let not_prefetched = self.0.prefetch(
-            &keys
-                .iter()
-                .filter_map(|key| match key {
-                    StoreKey::HgId(_) => {
-                        not_found.push(key.clone());
-                        None
-                    }
-                    StoreKey::Content(_, hgid) => match hgid {
-                        None => {
-                            not_found.push(key.clone());
-                            None
-                        }
-                        Some(hgid) => Some(StoreKey::hgid(hgid.clone())),
-                    },
-                })
-                .collect::<Vec<_>>(),
-        )?;
-
-        not_found.extend(not_prefetched);
-        Ok(not_found)
-    }
-
-    fn upload(&self, keys: &[StoreKey]) -> Result<Vec<StoreKey>> {
-        Ok(keys.to_vec())
-    }
-}
-
-impl HgIdDataStore for LfsFallbackRemoteStore {
-    fn get(&self, key: StoreKey) -> Result<StoreResult<Vec<u8>>> {
-        match self.prefetch(&[key.clone()]) {
-            Ok(_) => self.0.get(key),
-            Err(_) => Ok(StoreResult::NotFound(key)),
-        }
-    }
-
-    fn get_meta(&self, key: StoreKey) -> Result<StoreResult<Metadata>> {
-        match self.prefetch(&[key.clone()]) {
-            Ok(_) => self.0.get_meta(key),
-            Err(_) => Ok(StoreResult::NotFound(key)),
-        }
-    }
-
-    fn refresh(&self) -> Result<()> {
-        self.0.refresh()
-    }
-}
-
-impl LocalStore for LfsFallbackRemoteStore {
-    fn get_missing(&self, keys: &[StoreKey]) -> Result<Vec<StoreKey>> {
-        self.0.get_missing(keys)
-    }
-}
-
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 enum RetryStrategy {
     RetryError,