thermal/core: Remove unneeded EXPORT_SYMBOLS

Different functions are exporting the symbols but are actually only
used by the thermal framework internals. Remove these EXPORT_SYMBOLS.

Cc: Alexandre Bailon <abailon@baylibre.com>
Cc: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linexp.org>
Link: https://lore.kernel.org/r/20220722200007.1839356-6-daniel.lezcano@linexp.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>