layers: Simplify command buffer pool cleanup
layers: Simplify command buffer pool cleanup

We don't need to carefully pick through the pool's buffers, removing
them. We're about to throw away the whole container.

Signed-off-by: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools