Replace mount-options with mount where appropriate.

Since our minimum supported version is now 1.16 and mount was fixed in
1.13.16, it is now safe to replace mount-options + empty options with
mount wherever it occurs.

ruby/examples/create_disk.rb

@@ -42,7 +42,7 @@ end
 g.mkfs("ext4", partitions[0])
 
 # Now mount the filesystem so that we can add files.
-g.mount_options("", partitions[0], "/")
+g.mount(partitions[0], "/")
 
 # Create some files and directories.
 g.touch("/empty")