gbm: remove magma backend

Change-Id: I866a43088f030e812d3e46cb5102044b7e257893
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1348329
Reviewed-by: David Gilhooley <dgilhooley@google.com>
Commit-Queue: Craig Stout <cstout@google.com>
3 files changed