hw/rtc/ls7a_rtc: Drop unused inline functions

Remove toy_val_to_time_mon and toy_val_to_time_year as unused,
to avoid a build failure with clang.  Remove all of the other
inline markers too so that this does not creep back in.

Reviewed-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>