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/cache/cache_unsupported.go

@@ -1,6 +1,6 @@
 // Build for cache for unsupported platforms to stop go complaining
 // about "no buildable Go source files "
 
-// +build plan9 !go1.7
+// +build plan9
 
 package cache