internal: fix autosynth (#520)

It turns out autosynth creates the PR for you, so we don't have to
do all the extra bits. It should detect that there are local
changes and open a PR.

I am still not sure we will be using this automation in the
future. I would like to simplify things and move over to GH actions
eventually. Because of this I am leaving the code to submit a PR
in for now. I filed #519 to track clean up this code or switching
automation.
diff --git a/internal/kokoro/synth/synth.sh b/internal/kokoro/synth/synth.sh
index 6899bd6..dda6d99 100755
--- a/internal/kokoro/synth/synth.sh
+++ b/internal/kokoro/synth/synth.sh
@@ -6,37 +6,8 @@
 
 set -e -x
 
-WORKING_BRANCH=synth-update
-
 cd $(dirname $0)/../../..
 
-should_create_pr=1
-git checkout -b $WORKING_BRANCH || {
-    # The branch already exists - assume there's an existing PR.
-    unset should_create_pr;
-
-    # Reset back to master
-    git checkout $WORKING_BRANCH;
-    git reset --hard origin/master;
-}
-
 pushd google-api-go-generator
 make all
 popd
-
-if [[ -z "$(git status --porcelain)" ]]; then
-    echo "No changes today; exiting."
-    exit 0;
-fi
-
-echo "Pushing changes"
-
-commit_message="all: autogenerated update ($(date '+%Y-%m-%d'))"
-git commit --all -m "$commit_message"
-
-git push origin $WORKING_BRANCH:$WORKING_BRANCH -f
-
-if [ "$should_create_pr" ]; then
-    echo "Creating PR."
-    go run ./sendpr -title "$commit_message"
-fi