golang: Remove go1.5 and go1.6

Go1.5 and go1.6 are not used anywhere in slapos tree and neither these versions are supported by upstream release policy https://golang.org/doc/devel/release.html#policy anymore. So let's remove them.

component/golang/buildout.cfg

@@ -24,25 +24,6 @@ md5sum = d2cc61cb9f829b3510ee39c0c5568014
 environment-extra =
 
 
-[golang15]
-<= golang-common
-url = https://storage.googleapis.com/golang/go1.5.4.src.tar.gz
-md5sum = a04d570515c46e4935c63605cbd3a04e
-
-# go1.5 needs go1.4 to bootstrap
-environment-extra =
-  GOROOT_BOOTSTRAP=${golang14:location}
-
-
-[golang16]
-<= golang-common
-url = https://storage.googleapis.com/golang/go1.6.3.src.tar.gz
-md5sum = bf3fce6ccaadd310159c9e874220e2a2
-
-# go1.6 needs go1.4 to bootstrap
-environment-extra =
-  GOROOT_BOOTSTRAP=${golang14:location}
-
 [golang18]
 <= golang-common
 url = https://storage.googleapis.com/golang/go1.8.3.src.tar.gz