platform/chrome: cros_ec_lpc_mec: remove cros_ec_lpc_mec_destroy()

It's pointless (and invalid) to destroy a statically allocated mutex in
cros_ec_lpc_mec_destroy().

Let's remove it.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Link: https://lore.kernel.org/r/20221031050657.3899359-1-tzungbi@kernel.org