Remove builds conditional on go1.7 since that is now guaranteed #2154

Old fallback code was deleted and the go1.7 style code inlined where
appropriate.

backend/azureblob/azureblob_unsupported.go

@@ -1,6 +0,0 @@
-// Build for unsupported platforms to stop go complaining
-// about "no buildable Go source files "
-
-// +build !go1.7
-
-package azureblob