Remove third_party/golang.org/x/mobile as it is no longer used with Go 1.5.

third_party/go/src/golang.org/x/mobile/CONTRIBUTING.md

Index: third_party/go/src/golang.org/x/mobile/CONTRIBUTING.md
diff --git a/third_party/go/src/golang.org/x/mobile/CONTRIBUTING.md b/third_party/go/src/golang.org/x/mobile/CONTRIBUTING.md
deleted file mode 100644
index 88dff59bc7d200392543337d37013f2d89dde6df..0000000000000000000000000000000000000000
--- a/third_party/go/src/golang.org/x/mobile/CONTRIBUTING.md
+++ /dev/null
@@ -1,31 +0,0 @@
-# Contributing to Go
-
-Go is an open source project.
-
-It is the work of hundreds of contributors. We appreciate your help!
-
-
-## Filing issues
-
-When [filing an issue](https://golang.org/issue/new), make sure to answer these five questions:
-
-1. What version of Go are you using (`go version`)?
-2. What operating system and processor architecture are you using?
-3. What did you do?
-4. What did you expect to see?
-5. What did you see instead?
-
-General questions should go to the [golang-nuts mailing list](https://groups.google.com/group/golang-nuts) instead of the issue tracker.
-The gophers there will answer or ask you to file an issue if you've tripped over a bug.
-
-## Contributing code
-
-Please read the [Contribution Guidelines](https://golang.org/doc/contribute.html)
-before sending patches.
-
-**We do not accept GitHub pull requests**
-(we use [Gerrit](https://code.google.com/p/gerrit/) instead for code review).
-
-Unless otherwise noted, the Go source files are distributed under
-the BSD-style license found in the LICENSE file.
-