arm: Update UNUSED()

Fix previous commit - No need to check for GNUC/clang as always true
where UNUSED() is defined.

Signed-off-by: Peter Barada <peter.barada@gmail.com>

Author: pbarada

include/nuttx/compiler.h

@@ -625,11 +625,7 @@
 /* Indicate that a local variable is not used */
 
 #  ifndef UNUSED
-#    if defined(__clang__) || defined(__GNUC__)
 #      define UNUSED(a) (void)(a)
-#    else
-#      define UNUSED(a) ((void)(1 || &(a)))
-#    endif
 #  endif
 
 #  if defined(__clang__)