Comment out [[unlikely]] hints

The syntax seems to have changed and they don't really exist until
C++20. Probably not significant performance improvements anyway.
So they're commented out until proved useful, to suppress the
warnings.

include/opvcxx/ClockRecovery.h

@@ -104,11 +104,11 @@ class ClockRecovery
         int8_t offset = sample_index_ - prev_sample_index_;
 
         // When in spec, the clock should drift by less than 1 sample per frame.
-        if (offset >= MAX_OFFSET) [[unlikely]]
+        if (offset >= MAX_OFFSET) //!!! [[unlikely]]
         {
             offset -= SAMPLES_PER_SYMBOL;
         }
-        else if (offset <= -MAX_OFFSET) [[unlikely]]
+        else if (offset <= -MAX_OFFSET) //!!! [[unlikely]]
         {
             offset += SAMPLES_PER_SYMBOL;
         }
@@ -120,7 +120,7 @@ class ClockRecovery
     {
         // update_sample_index_() must be called first.
 
-        if (frame_count_ == 0) [[unlikely]]
+        if (frame_count_ == 0) //!!! [[unlikely]]
         {
             prev_sample_index_ = sample_index_;
             offset_ = 0.0;

include/opvcxx/OPVDemodulator.h

@@ -556,7 +556,7 @@ void OPVDemodulator<FloatType>::operator()(const FloatType input)
 
 	// We need to pump a few ms of data through on startup to initialize
 	// the demodulator.
-	if (initializing) [[unlikely]]
+	if (initializing) //!!! [[unlikely]]
 	{
 		--initializing;
 		initialize(input);