libspl: move kstat definitions from zfs_context.h, slim down to basics

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

@@ -182,18 +182,6 @@ extern "C" {
 typedef off_t loff_t;
 #endif
 
-/*
- * kstat creation, installation and deletion
- */
-extern kstat_t *kstat_create(const char *, int,
-    const char *, const char *, uchar_t, ulong_t, uchar_t);
-extern void kstat_install(kstat_t *);
-extern void kstat_delete(kstat_t *);
-extern void kstat_set_raw_ops(kstat_t *ksp,
-    int (*headers)(char *buf, size_t size),
-    int (*data)(char *buf, size_t size, void *data),
-    void *(*addr)(kstat_t *ksp, loff_t index));
-
 /*
  * procfs list manipulation
  */