Vulkan: Minor SecondaryCommandPool::destroy() optimization. Follow up for the commit: 9b6368ccfceddf1784ef41e95e57696453cca71d Vulkan: Fix freeing Secondary Command Buffers from wrong thread. Bug: angleproject:6100 Change-Id: I8ef34898941c0c5684d3ea4378332cdf6b87c0c8 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4359703 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Igor Nazarov <i.nazarov@samsung.com> Reviewed-by: Charlie Lao <cclao@google.com>