thermal: imx: Calling imx_thermal_unregister_legacy_cooling() in .remove

imx_thermal_unregister_legacy_cooling() should be used for handling
legacy cpufreq cooling cleanups in .remove callback instead of
calling cpufreq_cooling_unregister() and cpufreq_cpu_put() directly,
especially for !CONFIG_CPU_FREQ scenario, no operation needed for
handling legacy cpufreq cooling cleanups at all.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1584088094-24857-1-git-send-email-Anson.Huang@nxp.com