Remove note on marking copy_to fields as stored. (#65459)

During highlighting, we now load all values that were copied into the field
through copy_to. So there's no longer a reason to set 'store: true' to account
for fields not available in _source.

docs/reference/mapping/params/store.asciidoc

@@ -65,6 +65,3 @@ If you need the original value, you should retrieve it from the `_source`
 field instead.
 
 ======================================
-
-Another situation where it can make sense to make a field stored is for those
-that don't appear in the `_source` field (such as <<copy-to,`copy_to` fields>>).