Remove all traces of Dr Memory.

The Dr Memory toolchain is no longer supported by Chromium and
their bots have been removed. WebRTC will now rely on the LLVM
santizers for catching such errors.

BUG=webrtc:6553
NOTRY=True
R=ehmaldonado@webrtc.org

Review URL: https://codereview.webrtc.org/2434563003 .

Cr-Commit-Position: refs/heads/master@{#14703}

setup_links.py

@@ -38,7 +38,6 @@ DIRECTORIES = [
   'third_party/boringssl',
   'third_party/closure_compiler',
   'third_party/colorama',
-  'third_party/drmemory',
   'third_party/expat',
   'third_party/ffmpeg',
   'third_party/instrumented_libraries',