zfs_context: move empty __init/__exit macros to sys/debug.h

These are kind-of compiler attribute placeholders, so go here with the
others for now.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17861

include/sys/zfs_context.h

@@ -137,12 +137,6 @@ extern "C" {
 typedef off_t loff_t;
 #endif
 
-/*
- * Kernel modules
- */
-#define	__init
-#define	__exit
-
 #endif  /* _KERNEL || _STANDALONE */
 
 #ifdef __cplusplus