Remove unused rt_time_t

Author: lygstate

include/rttypes.h

@@ -88,7 +88,6 @@ typedef rt_ubase_t                       rt_uintptr_t;   /**< Type for unsigned
 #endif /* defined(RT_USING_LIBC) && !defined(RT_USING_NANO) */
 
 typedef rt_base_t                       rt_err_t;       /**< Type for error number */
-typedef rt_uint32_t                     rt_time_t;      /**< Type for time stamp */
 typedef rt_uint32_t                     rt_tick_t;      /**< Type for tick count */
 typedef rt_base_t                       rt_flag_t;      /**< Type for flags */
 typedef rt_ubase_t                      rt_dev_t;       /**< Type for device */