Revert "Slab-allocate VkDescriptorSets"

This reverts commit 96c4019214.

Reason for revert: Breaks the roll, WebGPU CTS hits the
ASSERT(totalDescriptorCount > 0)

Original change's description:
> Slab-allocate VkDescriptorSets
> 
> This introduces a slab allocator for VkDescriptorSets which creates
> a VkDescriptorPool pre-allocated with multiple VkDescriptorSets per
> BindGroupLayout. In the future, we can deduplicate pools that have
> the same, or roughly the same, descriptor counts.
> 
> This CL also removes the old DescriptorSetService and moves most of
> the functionality onto the DescriptorSetAllocator itself to keep
> the tracking logic in one place.
> 
> Bug: dawn:340
> Change-Id: I785b17f4353fb3d40c9ccc33746600d6794efe7c
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/19320
> Reviewed-by: Austin Eng <enga@chromium.org>
> Commit-Queue: Austin Eng <enga@chromium.org>

TBR=cwallez@chromium.org,kainino@chromium.org,enga@chromium.org,bryan.bernhart@intel.com

Change-Id: Icb58485f1080eab79b24fbcd834a89fc6206b80e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: dawn:340
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/20280
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/tests/end2end/DeprecatedAPITests.cpp

@@ -288,10 +288,6 @@ TEST_P(DeprecationTests, BGDescBindingAndEntriesBothZeroEmitsNoWarning) {
 
 // Test that creating a BG with bindings still does correct state tracking
 TEST_P(DeprecationTests, BGDescBindingStateTracking) {
-    // TODO(cwallez@chromium.org): In Vulkan it is disallowed to create 0-sized descriptor pools
-    // but the Vulkan backend doesn't special case it yet.
-    DAWN_SKIP_TEST_IF(IsVulkan());
-
     wgpu::BindGroupLayout layout = utils::MakeBindGroupLayout(device, {});
 
     // Test a case where if |bindings| wasn't taken into account, no validation error would happen