Removed all instances of crash_prog()

crash_prog was introduced to force-crash a userspace program
since _exit may not yet be implemented. However, the new versions
of OS161 already have the exact same logic as crash_prog in stdlib/exit.

userland/testbin/sparsefile/sparsefile.c

@@ -109,7 +109,5 @@ main(int argc, char *argv[])
 
 	nprintf("\n");
 	success(TEST161_SUCCESS, SECRET, "/testbin/sparsefile");
-	// Exit may not be implemented. So crash.
-	crash_prog();
 	return 0;
 }