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

backend/dropbox/dropbox_test.go

@@ -2,9 +2,6 @@
 //
 // Automatically generated - DO NOT EDIT
 // Regenerate with: make gen_tests
-
-// +build go1.7
-
 package dropbox_test
 
 import (

backend/dropbox/dropbox_unsupported.go

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