[mb] Remove redundant flags

BUG=chromium:616035
TBR=kjellander@chromium.org, vogelheim@chromium.org, jochen@chromium.org
NOTRY=true

Review-Url: https://codereview.chromium.org/2071313002
Cr-Commit-Position: refs/heads/master@{#37067}

infra/mb/mb_config.pyl

@@ -48,7 +48,6 @@
     },
 
     'debug': {
-      'gyp_defines': 'v8_enable_slow_dchecks=1 v8_optimized_debug=1',
       'gn_args': 'is_debug=true',
     },