Remove references from .gn to third_party/mesa

third_party/mesa is no longer part of Chromium, and having it
listed here enforces gn checks for any product that still
checks out mesa in third_party (i.e. Google Chrome) which is
a problem.

Bug: 873321
Change-Id: If1c28fb2c393f04c9b0fe0703204fd7225a1a483
Reviewed-on: https://chromium-review.googlesource.com/c/1333812
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607653}
diff --git a/.gn b/.gn
index 54471d2..8fb99ce 100644
--- a/.gn
+++ b/.gn
@@ -499,7 +499,6 @@
   "//third_party/markdown/*",
   "//third_party/markupsafe/*",
   "//third_party/material_design_icons/*",
-  "//third_party/mesa/*",
   "//third_party/mesa_headers/*",
   "//third_party/metrics_proto/*",
   "//third_party/minigbm/*",