xtensa: remove sys_define_gpr_with_alias()

This is not being used in Xtensa anywhere, so remove.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung

include/zephyr/arch/xtensa/arch.h

@@ -46,9 +46,6 @@
  * @}
  */
 
-/* Xtensa GPRs are often designated by two different names */
-#define sys_define_gpr_with_alias(name1, name2) union { uint32_t name1, name2; }
-
 #include <zephyr/arch/xtensa/exception.h>
 
 #ifdef __cplusplus