Routers and WiFi devices have to undergo certification to have the "WiFi" branding. Any router that does not have a 2.4 Ghz radio would be lacking support for IEEE 802.11 b/g/n and surely would fail certification.
That means there is no such thing as a "WiFi" router with only a 5 Ghz radio. You can disable the 2.4 Ghz radio in many routers though.
For now, I would not recommend seeking out a product that explicitly avoids having a 2.4 Ghz radio. It wouldn't be guaranteed to work with all 5 Ghz devices since the certification is not there. Some day the WiFi Alliance may decide to deprecate the 2.4 Ghz standards and this answer may no longer be correct.