Revert "terraform/environment: double queue task parallelization"

This reverts commit 39a90e473dedfa1b81b10bde98d1cd1b406e4eac.

Reason for revert: This creates more 429 failures.

Change-Id: I233f5416983cf2bedf505e4aafc077e3de05be59
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/539535
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
diff --git a/terraform/environment/worker.tf b/terraform/environment/worker.tf
index 4b719ee..ea7142d 100644
--- a/terraform/environment/worker.tf
+++ b/terraform/environment/worker.tf
@@ -213,7 +213,7 @@
   project  = var.project
 
   rate_limits {
-    max_concurrent_dispatches = 400
+    max_concurrent_dispatches = 200
     max_dispatches_per_second = 500
   }