Don't offer to fix indices until we can do it right

The issue in CRM-20817 is appearing on a lot of sites.  Most recently, I was updating a site we use on a demo computer that was installed in 4.7.17, used for two days, and sat in a box for six months.  Even it had the system message about missing indices, and even it failed when I clicked Update Indices on 4.7.25.

We should not be telling all users there's a problem and offering a solution unless that solution is reliable.

I'm proposing bypassing the system check (leaving the code in core for continued development) until CRM-20817 and CRM-20533 are 100% resolved.