Remove reference to poisoned avr-libc define

[leres]

avr-libc-1.8.0 "poisoned" SIG_USART1_RECV which causes issues when compiling with -Werror.

This patches HardwareSerial.cpp to avoid touching SIG_USART1_RECV. Also get rid of SIG_USART2_RECV and SIG_USART3_RECV while we're at it (plus it's silly to have a #elif defined() that just does #error!)

patch-hardware-arduino-cores-arduino-HardwareSerial.cpp

[cmaglie]

Fixed, it will be available on next releases (1.0.5 and 1.5.3)