[mesa] Delete reference to `submodules` GN arg

Now that we rolled back submodules in fuchsia.git, the `submodules` GN
arg will never be true, and `.git` directories will go back to always
being located within their repositories rather than under the root
`.git/modules` directory.

Bug: 406021557
Change-Id: I0fb1cfad7326173b6cad951b71dc03a41ed96751
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1235248
Fuchsia-Auto-Submit: Oliver Newman <olivernewman@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Craig Stout <cstout@google.com>