Remove ExceptionCodePlaceholder

This class is no longer used

BUG=252233
TBR=abarth

Review URL: https://chromiumcodereview.appspot.com/22241002

git-svn-id: svn://svn.chromium.org/blink/trunk@155539 bbb929c8-8fbe-4397-9dbb-9b2b20218538