libraries/SPI: remove pointless abs(...) call

SPI library code erroneously assumed that:
- abs() is a C function, so include stdlib.h is required.
  what happens instead is Arduino.h shadows `abs()` with it's own macro
- uint32_t() - int32_t() promotes to int32_t, thus needing abs()

Fix both issues, leaving existing calculations as-is.

Author: mcspr

libraries/SPI/SPI.cpp

@@ -260,7 +260,7 @@ void SPIClass::setFrequency(uint32_t freq) {
                 break;
             } else if(calFreq < (int32_t) freq) {
                 // never go over the requested frequency
-                if(abs(freq - calFreq) < abs(freq - bestFreq)) {
+                if((freq - calFreq) < (freq - bestFreq)) {
                     bestFreq = calFreq;
                     memcpy(&bestReg, &reg, sizeof(bestReg));
                 }

libraries/SPI/SPI.h

@@ -22,7 +22,6 @@
 #define _SPI_H_INCLUDED
 
 #include <Arduino.h>
-#include <stdlib.h>
 
 #define SPI_HAS_TRANSACTION 1