INSTALL file needs to be update

Check for Drupal 9 compatibility.
The INSTALL file contains a few functions that need to be updated as per Drupal 9 compatibility.

Find below the list of functions that need to be updated

------ -------------------------------------------------------
Line simple_node_importer.install
------ -------------------------------------------------------
21 Call to deprecated function entity_delete_multiple().
33 Call to deprecated function db_table_exists().
34 Call to deprecated function db_drop_table().
44 Call to deprecated function db_table_exists().
------ -------------------------------------------------------

If any missed out, fix those also.