build: Remove Appveyor and old CircleCI stuff

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/2997

script/circle-installgo.sh

@@ -1,33 +0,0 @@
-#!/bin/bash
-set -euo pipefail
-
-[ -d ~/go1.5 ] && exit
-
-# Install the version of Go that we want
-
-curl -s https://storage.googleapis.com/golang/go1.5.linux-amd64.tar.gz \
-	| tar -C ~ --transform s/go/go1.5/ -zx
-
-# Build the standard library for all our cross compilation targets. We do that
-# here so that it gets cached and we don't need to repeat it for every build.
-
-for GOOS in darwin dragonfly solaris; do
-	export GOOS
-	export GOARCH=amd64
-	echo $GOOS $GOARCH
-	go install std
-done
-
-for GOOS in freebsd linux netbsd openbsd windows; do
-	for GOARCH in amd64 386; do
-		export GOOS
-		export GOARCH
-		echo $GOOS $GOARCH
-		go install std
-	done
-done
-
-export GOOS=linux
-export GOARCH=arm
-echo $GOOS $GOARCH
-go install std