GN build: remove Linux support

No longer used.

Change-Id: I4b0026fbe3092e39bc2e6569928256710f1dcb22
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/982951
Reviewed-by: John Bauman <jbauman@google.com>
Commit-Queue: Craig Stout <cstout@google.com>
6 files changed