Migrate to go1.12 to simplify our cmd/http package (#7302)

Simplify the cmd/http package overall by removing
custom plain text v/s tls connection detection, by
migrating to go1.12 and choose minimum version
to be go1.12

Also remove all the vendored deps, since they
are not useful anymore.

buildscripts/cross-compile.sh

@@ -11,7 +11,7 @@ function _init() {
     SUPPORTED_OSARCH="linux/ppc64le linux/arm64 linux/s390x darwin/amd64 freebsd/amd64"
 }
 
-function _build_and_sign() {
+function _build() {
     local osarch=$1
     IFS=/ read -r -a arr <<<"$osarch"
     os="${arr[0]}"
@@ -22,13 +22,14 @@ function _build_and_sign() {
     # Go build to build the binary.
     export GOOS=$os
     export GOARCH=$arch
+    export GO111MODULE=on
     go build -tags kqueue -o /dev/null
 }
 
 function main() {
     echo "Testing builds for OS/Arch: ${SUPPORTED_OSARCH}"
     for each_osarch in ${SUPPORTED_OSARCH}; do
-        _build_and_sign "${each_osarch}"
+        _build "${each_osarch}"
     done
 }