Simplify FormKeyGenerator::willDeleteForm.
Simplify FormKeyGenerator::willDeleteForm.

We had it->second access before wkrev.com/123691. wkrev.com/123691
removed the it->second access, and we can make the function just
m_formToKeyMap.remove(form).

BUG=none
TEST=none; no behavior changes

Review URL: https://chromiumcodereview.appspot.com/16355008

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@152274 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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