Revert "[Tools] Pass --delete_unversioned_trees to gclient sync."

Doing this is necessary at least temporarily, otherwise the bots always
have problems when moving to M39 because third_party/speex became part
of Chromium itself.

For more information, see
https://groups.google.com/a/chromium.org/d/msg/chromium-dev/-ozqcZSjd5k/F58XSTZUp7AJ

This reverts commit 522741d7d2c169cbe09cdcfd62b991d8d3ff98ca.