Revert incorrect/redundant fix for old BUG#34656

The real bug was that open_tables() returned error in case of
thd->killed() without properly calling thd->send_kill_message()
to set the correct error. This was fixed some time ago.

So remove the, now redundant, extra checks for thd->is_error(),
possibly allowing to catch in debug builds more incorrect
error handling cases.

sql/sql_show.cc

@@ -5285,14 +5285,11 @@ err:
     const char *error= thd->is_error() ? thd->get_stmt_da()->message() : "";
     table->field[20]->store(error, strlen(error), cs);
 
-    if (thd->is_error())
-    {
     push_warning(thd, Sql_condition::WARN_LEVEL_WARN,
                  thd->get_stmt_da()->sql_errno(),
                  thd->get_stmt_da()->message());
     thd->clear_error();
   }
-  }
 
   DBUG_RETURN(schema_table_store_record(thd, table));
 }
@@ -5452,7 +5449,6 @@ static int get_schema_column_record(THD *thd, TABLE_LIST *tables,
         I.e. we are in SELECT FROM INFORMATION_SCHEMA.COLUMS
         rather than in SHOW COLUMNS
       */
-      if (thd->is_error())
       push_warning(thd, Sql_condition::WARN_LEVEL_WARN,
                    thd->get_stmt_da()->sql_errno(),
                    thd->get_stmt_da()->message());