dm: spi: Clean up detection of sandbox SPI emulator

Now that we don't have to deal with the command-line flag we can simplify
the code for detecting the emulator. Remove the lookup based on the SPI
specification, relying just on the device tree to locate the emulator.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/include/asm/state.h

@@ -36,7 +36,6 @@ enum state_terminal_raw {
 };
 
 struct sandbox_spi_info {
-	const char *spec;
 	struct udevice *emul;
 };