[arch][arm64] remove some extraneous copy-pasta in the ops header

arch/arm64/include/arch/arch_ops.h

@@ -46,7 +46,6 @@ static inline void arch_disable_fiqs(void) {
     CF;
 }
 
-// XXX
 static inline bool arch_fiqs_disabled(void) {
     unsigned int state;
 
@@ -71,23 +70,8 @@ static inline bool arch_fiqs_disabled(void) {
 #endif
 
 static inline uint32_t arch_cycle_count(void) {
-#if ARM_ISA_ARM7M
-#if ENABLE_CYCLE_COUNTER
-#define DWT_CYCCNT (0xE0001004)
-    return *REG32(DWT_CYCCNT);
-#else
-    return 0;
-#endif
-#elif ARM_ISA_ARMV7
-    uint32_t count;
-    __asm__ volatile("mrc       p15, 0, %0, c9, c13, 0"
-                     : "=r" (count)
-                    );
-    return count;
-#else
 //#warning no arch_cycle_count implementation
     return 0;
-#endif
 }
 
 /* use the cpu local thread context pointer to store current_thread */