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: refs/heads/master@{#378200}

docs/mac_build_instructions.md

@@ -24,8 +24,7 @@
 ## Getting the code
 
 [Check out the source code](http://dev.chromium.org/developers/how-tos/get-the-code)
-using Git. If you're new to the project, you can skip all the information about
-git-svn, since you will not be committing directly to the repository.
+using Git.
 
 Before checking out, go to the
 [waterfall](http://build.chromium.org/buildbot/waterfall/) and check that the