internal: drop support for 1.9 and 1.10

As of Oct 1, 2019 GCP is no longer supporting Go 1.9 and 1.10.
Cleaning up build scripts to support this.

Change-Id: Ifb71c17a1961cac27cb3834172ceeb7c97d88c2a
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/46051
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>
diff --git a/internal/kokoro/test.sh b/internal/kokoro/test.sh
index 37a5102..5e98c66 100755
--- a/internal/kokoro/test.sh
+++ b/internal/kokoro/test.sh
@@ -17,6 +17,7 @@
 export GOPATH="$HOME/go"
 export GOCLOUD_HOME=$GOPATH/src/google.golang.org/api/
 export PATH="$GOPATH/bin:$PATH"
+export GO111MODULE=on
 mkdir -p $GOCLOUD_HOME
 
 # Move code into $GOPATH and get dependencies
@@ -25,19 +26,8 @@
 
 try3() { eval "$*" || eval "$*" || eval "$*"; }
 
-download_deps() {
-    if [[ `go version` == *"go1.11"* ]] || [[ `go version` == *"go1.12"* ]]; then
-        export GO111MODULE=on
-        # All packages, including +build tools, are fetched.
-        try3 go mod download
-    else
-        # Because we don't provide -tags tools, the +build tools
-        # dependencies aren't fetched.
-        try3 go get -v -t ./...
-    fi
-}
-
-download_deps
+# All packages, including +build tools, are fetched.
+try3 go mod download
 ./internal/kokoro/vet.sh
 
 # Run tests and tee output to log file, to be pushed to GCS as artifact.