[build] Remove unused __keepme macro

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/compiler.h

  */
 #define __weak		__attribute__ (( weak, noinline ))
 
-/** Prevent a function from being optimized away without inlining
- *
- * Calls to functions with void return type that contain no code in their body
- * may be removed by gcc's optimizer even when inlining is inhibited. Placing
- * this macro in the body of the function prevents that from occurring.
- */
-#define __keepme	asm("");
-
 #endif
 
 /** @defgroup dbg Debugging infrastructure