Remove use of OS provided random functions

We added the internal random generator a long time ago, and kept
the OS variant around as an opt-in feature with using use_os_rand=1.
We defaulted to using the fio provided one, and I doubt that
anyone has used the option.

The time has come to kill it.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/options.c b/options.c
index 918de8e..529b8f0 100644
--- a/options.c
+++ b/options.c
@@ -1842,12 +1842,8 @@
 	{
 		.name	= "use_os_rand",
 		.lname	= "Use OS random",
-		.type	= FIO_OPT_BOOL,
-		.off1	= td_var_offset(use_os_rand),
-		.help	= "Set to use OS random generator",
-		.def	= "0",
-		.parent = "rw",
-		.hide	= 1,
+		.type	= FIO_OPT_DEPRECATED,
+		.off1	= td_var_offset(dep_use_os_rand),
 		.category = FIO_OPT_C_IO,
 		.group	= FIO_OPT_G_RANDOM,
 	},