[DOCS] Remove metrics sidebar in `_source` docs (#60777)

docs/reference/mapping/fields/source-field.asciidoc

@@ -51,21 +51,6 @@ and <<docs-reindex,`reindex`>> APIs.
 TIP: If disk space is a concern, rather increase the
 <<index-codec,compression level>> instead of disabling the `_source`.
 
-.The metrics use case
-**************************************************
-
-The _metrics_ use case is distinct from other time-based or logging use cases
-in that there are many small documents which consist only of numbers, dates,
-or keywords.  There are no updates, no highlighting requests, and the data
-ages quickly so there is no need to reindex.  Search requests typically use
-simple queries to filter the dataset by date or tags, and the results are
-returned as aggregations.
-
-In this case, disabling the `_source` field will save space and reduce I/O.
-
-**************************************************
-
-
 [[include-exclude]]
 ==== Including / Excluding fields from `_source`