remove ambiguous TTL from glossary

- The current TTL in glossary the specifically refers to the row-level time-to-live feature
- Removal prevents confusion with other TTL concepts in documentation

glossary.md

@@ -172,7 +172,3 @@ Top SQL helps locate SQL queries that contribute to a high load of a TiDB or TiK
 ### TSO
 
 Because TiKV is a distributed storage system, it requires a global timing service, Timestamp Oracle (TSO), to assign a monotonically increasing timestamp. In TiKV, such a feature is provided by PD, and in Google [Spanner](http://static.googleusercontent.com/media/research.google.com/en//archive/spanner-osdi2012.pdf), this feature is provided by multiple atomic clocks and GPS.
-
-### TTL
-
-[Time to live (TTL)](/time-to-live.md) is a feature that allows you to manage TiDB data lifetime at the row level. For a table with the TTL attribute, TiDB automatically checks data lifetime and deletes expired data at the row level.