The ANGLE Vulkan backend is now thread safe for non-share group contexts. This
means that a global GL lock only adds overhead for most use cases.

Remove the angle_force_thread_safety gn argument.

BUG=angleproject:2464

Change-Id: Ic6ba89e18b46e5dd72aa83d0f409097441fcca3a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1635749
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>