Deprecate Database::flush() function

The Database::flush() function is only needed for the sled database on mobile, instead for mobile use the sqlite database.

src/database/mod.rs

@@ -159,6 +159,10 @@ pub trait Database: BatchOperations {
     /// It should insert and return `0` if not present in the database
     fn increment_last_index(&mut self, keychain: KeychainKind) -> Result<u32, Error>;
 
+    #[deprecated(
+        since = "0.18.0",
+        note = "The flush function is only needed for the sled database on mobile, instead for mobile use the sqlite database."
+    )]
     /// Force changes to be written to disk
     fn flush(&mut self) -> Result<(), Error>;
 }