sam: Optimized delayMicroseconds() (Rob Tillaart)

See #1121
Removed unused stuff from WiFi library.
Removed unused "socket.h" interface from WiFi library.

(that was probably an heritage from Ethernet lib)
Moved vanilla parts of WiFi library into common source folder.
WiFi librari: SPI delay is computed based on F_CPU constant
WiFi library is now vanilla