Remove linux_memcheck from default trybots.

Now that we have enabled leak detection (LeakSanitizer) for our
Linux ASan bots, there's no point always running the linux_memcheck trybot
since it's very slow and almost always gets a long queue of builds.

BUG=
R=niklas.enbom@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/28479004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7203 4adac7df-926f-26a2-2b94-8c16560cd09d