Oilpan: Remove manual finalization of DatabaseContext::m_openSyncDatabases.

Blink r170443 [1] fixed a bug of HeapHashMap. We don't need to clear OwnPtrs
manually.

[1] https://src.chromium.org/viewvc/chrome?view=rev&revision=170443

BUG=347902,357113

Review URL: https://codereview.chromium.org/223453002

git-svn-id: svn://svn.chromium.org/blink/trunk@170757 bbb929c8-8fbe-4397-9dbb-9b2b20218538