diagnose_bloat.py: Remove symbol_level=0 now that it's required

Bug: 648475

Change-Id: Ibbafd96a020850881c0c515e0e4dd71ee530c099
Reviewed-on: https://chromium-review.googlesource.com/1187922Reviewed-by: Peter Collingbourne <pcc@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585682}

tools/binary_size/diagnose_bloat.py

@@ -313,7 +313,6 @@ class _BuildHelper(object):
 
   def _GenGnCmd(self):
     gn_args = 'is_official_build=true'
-    gn_args += ' symbol_level=0'
     # Variables often become unused when experimenting with macros to reduce
     # size, so don't fail on warnings.
     gn_args += ' treat_warnings_as_errors=false'