docs: remove some old urls from useful_urls.md file
docs: remove some old urls from useful_urls.md file

This patch removes two old urls from the Subversion (svn) and Git-SVN
eras that are not relevant anymore.

Instead point to the new Source Code Browser, called Gitiles, that we
use now in the pure Git world.

BUG=None
R=nodir@chromium.org

Review URL: https://codereview.chromium.org/1749213003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#378725}

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