thermal/governor: Remove deprecated information

The userspace governor is still in use on production systems and the
deprecating warning is scary.

Even if we want to get rid of the userspace governor, it is too soon
yet as the alternatives are not yet adopted.

Change the deprecated warning by an information message suggesting to
switch to the netlink thermal events.

Fixes: 0275c9fb

 ("thermal/core: Make the userspace governor deprecated")
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>