thermal: don't make THERMAL_NETLINK 'default y'

We just don't do that.  "default y" is for things that are needed for
compatibility (when an old feature is made unconditional), or for things
that are basically part of the infrastructure of a platform.

And it can possibly be used for questions that don't enable code on
their own, but are used to enable or disable a whole slew of other
questions.

A new feature that people aren't using is never 'default y', unless it
cures cancer or ends world hunger.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>