Linux: remove use_system_mesa bits.

This doesn't seem useful, see https://groups.google.com/a/chromium.org/d/msg/chromium-packagers/amyyjkDj7Cg/cl5up-ABW7oJ

BUG=none
R=piman@chromium.org

Review URL: https://codereview.chromium.org/22218003

git-svn-id: http://src.chromium.org/svn/trunk/src/build@215695 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -848,9 +848,6 @@
     'spdy_proxy_auth_value%': '<(spdy_proxy_auth_value)',
     'enable_mdns%' : '<(enable_mdns)',
 
-    # Use system mesa instead of bundled one.
-    'use_system_mesa%': 0,
-
     # Use system nspr instead of the bundled one.
     'use_system_nspr%': 0,