spdk_top: reduce threads pop-up height

The threads pop-up has additional whitespace printed after the last
poller record. Since this empty space is not used anywhere in
in spdk_top, it can be removed.

Fixes #2137

Change-Id: I2e8fee79eea8ffd2a98e803cf41baa2fc4f95d24
Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/9360


Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>