[DOCS] Remove 7.8.0 coming tags (#58047)

Author: jrodewig

docs/reference/migration/migrate_7_8.asciidoc

@@ -9,8 +9,6 @@ your application to {es} 7.8.
 
 See also <<release-highlights>> and <<es-release-notes>>.
 
-coming[7.8.0]
-
 * <<breaking_78_aggregation_changes>>
 * <<breaking_78_mappings_changes>>
 * <<breaking_78_settings_changes>>