SD: remove any reference to pinMode(10, OUTPUT) in the examples

correct SS pin setup is already handled by SPI subsystem.
this should prevent future issues like #2868

current implementation assures that:
* pin10 is OUTPUT HIGH if SPI.begin() is called and the pin was unconfigured
* pin10 state is not modified if pinMode(10, OUTPUT) is called before SPI.begin()
* pin10 is INPUT HI-Z if nor pinMode(10, OUTPUT) nor SPI.begin() are called

libraries/SD/examples/ReadWrite/ReadWrite.ino

@@ -33,11 +33,6 @@ void setup()
 
 
   Serial.print("Initializing SD card...");
-  // On the Ethernet Shield, CS is pin 4. It's set as an output by default.
-  // Note that even if it's not used as the CS pin, the hardware SS pin
-  // (10 on most Arduino boards, 53 on the Mega) must be left as an output
-  // or the SD library functions will not work.
-  pinMode(10, OUTPUT);
 
   if (!SD.begin(4)) {
     Serial.println("initialization failed!");