Remove extend_lifespan method

Summary: Now that lifetime extension is provided by virtue of `labels` in snapshots, there isn't any need to have the `extend_lifespan` method which was supposed to manually push the expiry date every time it is called. More context in this quip: https://fb.quip.com/FO2RAtu5ZHb7

Reviewed By: yancouto

Differential Revision: D41520142

fbshipit-source-id: 7c9dad6a097548edb52bb735b41795097e239b30

Author: RajivTS

eden/mononoke/blobstore/ephemeral_blobstore/src/bubble.rs

@@ -426,10 +426,6 @@ impl Bubble {
             repo_config,
         }
     }
-
-    pub async fn extend_lifespan(&self) -> Result<()> {
-        unimplemented!()
-    }
 }
 
 // These blobstore methods are not to be used directly as they bypass redaction.

eden/mononoke/blobstore/ephemeral_blobstore/src/store.rs

@@ -47,8 +47,7 @@ struct RepoEphemeralStoreInner {
     pub(crate) connections: SqlConnections,
 
     /// Initial value of the lifespan for bubbles in this store, i.e. the
-    /// amount of time they last from either the call to create or the last
-    /// call to extend_lifespan.
+    /// amount of time they last from the call to create.
     pub(crate) initial_bubble_lifespan: ChronoDuration,
 
     /// Grace period after bubbles expire during which requests which have