Clean up duplicate of avr/pgmspace.h

pgmspace.h is provided by ArduinoCore-API and the core's included
copy was already shadowed by it. The copies were identical,
except for vsnprintf_P which was moved to silabs_additional.h.

Author: silabs-bozont

cores/gecko/avr/pgmspace.h

@@ -32,6 +32,8 @@
 #include "Arduino.h"
 #include "em_common.h"
 
+#define vsnprintf_P(str, size, format, ap) vsnprintf(str, size, format, ap)
+
 typedef enum {
   CPU_39MHZ,
   CPU_76MHZ,