Nuvoton: Remove dead code nu_delay_cycle_x4(...)

Originally, nu_delay_cycle_x4(...) is borrowed from mbed test code for delay
cycle. Currently, it is not used on Nuvoton targets. If delay cycle is needed,
use wait_ns(...) instead which has strict implementation and has passed tests.

targets/TARGET_NUVOTON/nu_timer.c

@@ -65,65 +65,3 @@ void nu_busy_wait_us(uint32_t us)
         prev = cur;
     }
 }
-
-/* Delay 4 cycles per round by hand-counting instruction cycles
- *
- * The delay function here is implemented by just hand-counting instruction cycles rather than preferred
- * H/W timer since it is to use in cases where H/W timer is not available. Usually, it can delay at least
- * 4-cycles per round.
- *
- * In modern pipeline core, plus flash performance and other factors, we cannot rely accurately on hand-
- * counting instruction cycles for expected delay cycles.
- */
-#if defined(__CC_ARM)
-MBED_NOINLINE
-__asm void nu_delay_cycle_x4(uint32_t rounds)
-{
-// AStyle should not format inline assembly
-// *INDENT-OFF*
-1
-#if !defined(__CORTEX_M0)
-    NOP                         // 1 cycle
-#endif
-    SUBS a1, a1, #1             // 1 cycle
-    BCS  %BT1                   // 3 cycles(M0)/2 cycles(non-M0)
-    BX   lr
-// *INDENT-ON*
-}
-#elif defined (__ICCARM__)
-MBED_NOINLINE
-void nu_delay_cycle_x4(uint32_t rounds)
-{
-    __asm volatile(
-        "loop: \n"
-#if !defined(__CORTEX_M0)
-        " NOP \n"               // 1 cycle
-#endif
-        " SUBS %0, %0, #1 \n"   // 1 cycle
-        " BCS.n  loop\n"        // 3 cycles(M0)/2 cycles(non-M0)
-        : "+r"(rounds)
-        :
-        : "cc"
-    );
-}
-#elif  defined ( __GNUC__ ) ||  (defined(__ARMCC_VERSION) && (__ARMCC_VERSION >= 6010050))
-MBED_NOINLINE
-void nu_delay_cycle_x4(uint32_t rounds)
-{
-    __asm__ volatile(
-        "%=:\n\t"
-#if !defined(__CORTEX_M0)
-        "NOP\n\t"               // 1 cycle
-#endif
-#if defined(__thumb__) && !defined(__thumb2__) && !defined(__ARMCC_VERSION)
-        "SUB  %0, #1\n\t"       // 1 cycle
-#else
-        "SUBS %0, %0, #1\n\t"   // 1 cycle
-#endif
-        "BCS  %=b\n\t"          // 3 cycles(M0)/2 cycles(non-M0)
-        : "+l"(rounds)
-        :
-        : "cc"
-    );
-}
-#endif

targets/TARGET_NUVOTON/nu_timer.h

@@ -74,9 +74,6 @@ void nu_countdown_free(struct nu_countdown_ctx_s *ctx);
  */
 void nu_busy_wait_us(uint32_t us);
 
-/* Delay 4 cycles per round by hand-counting instruction cycles */
-void nu_delay_cycle_x4(uint32_t rounds);  
-
 #ifdef __cplusplus
 }
 #endif