Remove saml_sp_request_tracking_cache from hook_schema

saml_sp_request_tracking_cache is a table that was removed in saml_sp_update_7207 but it's still in saml_sp_schema.

This can be an issue when using the site_test module since it tries to clone every table in hook_schema by selecting from the table and inserting into a copy. There is no source table since it was dropped in 7207.

Having the table in saml_sp_schema() also means new installs of the module get the table, but it might not hurt anything if it's not used.