Remove complicated libspl assert wrappers

Effectively provide our own version of assert()/verify() for use
in user space.  This minimizes our dependencies and aligns the
user space assertion handling with what's used in the kernel.

Signed-off-by: Carlo Landmeter <clandmeter@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4449

include/libzfs_impl.h

@@ -46,11 +46,6 @@
 extern "C" {
 #endif
 
-#ifdef	VERIFY
-#undef	VERIFY
-#endif
-#define	VERIFY	verify
-
 typedef struct libzfs_fru {
 	char *zf_device;
 	char *zf_fru;