add notes for removed variable

Author: idoqo

docs/process/v2_to_v3_environment_variables.md

@@ -22,6 +22,7 @@ Below is a list of affected variables and their new names.
 | `PERCONA_TEST_AUTH_HOST`                      |                                            | Removed in PMM v3, use `PMM_DEV_PERCONA_PLATFORM_ADDRESS`    |
 | `PERCONA_TEST_CHECKS_FILE`                    | `PMM_DEV_ADVISOR_CHECKS_FILE`              |                                                              |
 | `PERCONA_TEST_CHECKS_HOST`                    |                                            | Removed in PMM v3, use `PMM_DEV_PERCONA_PLATFORM_ADDRESS`    |
+| `PERCONA_TEST_CHECKS_INTERVAL`                |                                            | Removed in PMM v3 as it wasn't actually used.                |
 | `PERCONA_TEST_CHECKS_PUBLIC_KEY`              |                                            | Removed in PMM v3, use `PMM_DEV_PERCONA_PLATFORM_PUBLIC_KEY` |
 | `PERCONA_TEST_NICER_API`                      | `PMM_NICER_API`                            |                                                              |
 | `PERCONA_TEST_PMM_CLICKHOUSE_ADDR`            | `PMM_CLICKHOUSE_ADDR`                      |                                                              |