mb_config.pyl: Remove is_debug=false from official_optimize

It's redundant with is_official_build=true

Bug: None
Change-Id: I43ac78c75bb744c920da0e340285bc356d1316c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2327144Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794844}

tools/mb/mb_config.pyl

@@ -2854,7 +2854,7 @@
     },
 
     'official_optimize': {
-      'gn_args': 'is_official_build=true is_debug=false',
+      'gn_args': 'is_official_build=true',
     },
 
     'official_optimize_goma': {