Remove sentinal GLContext used in early days of Vulkan in our tools

I've ran this through multiple variations of dm and nanobench on Linux
and no longer see any crash/hangs. I forget what the original repo case
for the bug was, but I'm at least not seeing it now with updated drivers.

BUG=skia:

Change-Id: I6b7129a4c4d67938baa35d2e2c720cb078fc4c18
Reviewed-on: https://skia-review.googlesource.com/7441
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed