Fix waveform missing edges on cycle rollover

When the ESP cycle counter rolls over, the "now" can be smaller than the
next-edge time of a waveform generator.  This would cause the edge to be
missed on that specific pin, and make it look like PWM was hung.

Use proper comparison between current time and edge time.

Fixes esp8266#4944

Also remove the "sigma-delta.c.unused" file which was replaced by a
working one some time ago.

Author: earlephilhower

cores/esp8266/core_esp8266_sigma_delta.c.unused

@@ -102,13 +102,6 @@ static inline ICACHE_RAM_ATTR uint32_t min_u32(uint32_t a, uint32_t b) {
   return b;
 }
 
-static inline ICACHE_RAM_ATTR uint32_t min_s32(int32_t a, int32_t b) {
-  if (a < b) {
-    return a;
-  }
-  return b;
-}
-
 static inline ICACHE_RAM_ATTR void ReloadTimer(uint32_t a) {
   // Below a threshold you actually miss the edge IRQ, so ensure enough time
   if (a > 32) {
@@ -251,7 +244,8 @@ static ICACHE_RAM_ATTR void timer1Interrupt() {
 
       // Check for toggles
       now = GetCycleCount();
-      if (now >= wave->nextServiceCycle) {
+      int32_t cyclesToGo = wave->nextServiceCycle - now;
+      if (cyclesToGo < 0) {
         wave->state = !wave->state;
         if (wave->state) {
           SetGPIO(wave->gpioMask);