Remove an unused variable 'channel'

[estshorter]

I fixed the following compilation warning.

C:\Users\estshorter\.platformio\packages\framework-arduinoespressif32\cores\esp32\esp32-hal-rmt.c: In function 'rmtLoop':
C:\Users\estshorter\.platformio\packages\framework-arduinoespressif32\cores\esp32\esp32-hal-rmt.c:244:9: warning: unused variable 'channel' [-Wunused-variable]
     int channel = rmt->channel;
         ^

Note: This is already fixed in the idf-release/v4.0 and idf-release/v4.2 branches.

arduino-esp32/cores/esp32/esp32-hal-rmt.c

Lines 237 to 249 in d011dd7

	bool rmtLoop(rmt_obj_t* rmt, rmt_data_t* data, size_t size)
	{
	if (!rmt) {
	return false;
	}
	int allocated_size = MAX_DATA_PER_CHANNEL * rmt->buffers;
	if (size > allocated_size) {
	return false;
	}
	return _rmtSendOnce(rmt, data, size, true);
	}