Remove get_grub2_index() because it isn't used anywhere.

Link: http://lkml.kernel.org/r/20190509213647.6276-6-msys.mizuma@gmail.comSigned-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>