docs: get rid of git-svn note
docs: get rid of git-svn note

This patch removes from the mac build instructions the note about git-svn
era where Chromium was in subversion repo, and committers had to use git-svn
credentials to commit back to it when using git client. This is not relevant
anymore since the repo where Chromium now lives is a pure git repository, so
the git-svn commands are not necessary anymore and this comment can be just
deleted.

BUG=None
R=sdefresne@chromium.org

Review URL: https://codereview.chromium.org/1727363008

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#378200}

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