Go's policy is to only support the past two releases (which is
currently Go 1.11 and Go 1.10). But because App Engine was stuck on Go
1.6 and Go 1.8 for so long, we kept kinda supporting Go 1.6 anyway,
even though we didn't actively test it with any CI system.

But that led to code getting disgusting and full of too many
+build-tagged files and indirection, as this change shows.

So, remove Go 1.8, Go 1.7, and Go 1.6 support. We still "support" Go
1.9 for now, even though it's also not actively tested.

Fixes golang/go#26302

Change-Id: I4aa5793173e50ffcd67be52a464492ca48fc9a23
Reviewed-on: https://go-review.googlesource.com/c/145677Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>