Remove gltestthreading config

This hasn't found a real bug, ever. Instead, it's just a constant source
of flaky low-bit diffs that need to be excluded from testing.

Change-Id: Ib869fd40befdab5824d1429591082a1ceb718a94
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/726456
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
7 files changed