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/dropbox/dropbox.go

@@ -1,7 +1,4 @@
 // Package dropbox provides an interface to Dropbox object storage
-
-// +build go1.7
-
 package dropbox
 
 // FIXME dropbox for business would be quite easy to add