Drop dead description column from documents

The description field on Document and EmployeeDocument was never
populated. Remove it from the database, GraphQL schema, and Go types.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/coredata/migrations/20260407T120000Z.sql

@@ -14,3 +14,5 @@
 
 -- TODO: drop the title column from documents.
 ALTER TABLE documents ALTER COLUMN title DROP NOT NULL;
+
+ALTER TABLE documents DROP COLUMN description;