xtensa: drop set_except_vector declaration

There's no implementation for set_except_vector function in the xtensa
code. Drop its declaration.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/uapi/asm/setup.h

@@ -14,6 +14,4 @@
 
 #define COMMAND_LINE_SIZE	256
 
-extern void set_except_vector(int n, void *addr);
-
 #endif