Revert "[Reflection] Add API for inspecting async task allocation slabs."

[compnerd]

Reverts #40071

This causes use of uninitialized memory on Windows. https://ci-external.swift.org/job/oss-swift-windows-x86_64-vs2019/7174/

[compnerd]

@swift-ci please test

[compnerd]

Local testing (in conjunction with @mikeash) indicates that removal of the metadata field does "repair" the failure.

diff --git a/stdlib/public/runtime/StackAllocator.h b/stdlib/public/runtime/StackAllocator.h
index bbcc2a01e9b..1229ad99725 100644
--- a/stdlib/public/runtime/StackAllocator.h
+++ b/stdlib/public/runtime/StackAllocator.h
@@ -89,10 +89,6 @@ private:
   /// This struct is actually just the slab header. The slab buffer is tail
   /// allocated after Slab.
   struct Slab {
-    /// A fake metadata pointer that analysis tools can use to identify slab
-    /// allocations.
-    const void *metadata;
-
     /// A single linked list of all allocated slabs.
     Slab *next = nullptr;

@@ -103,7 +99,7 @@ private:
     // Here starts the tail allocated memory buffer of the slab.

     Slab(size_t newCapacity)
-        : metadata(SlabMetadataPtr), capacity(newCapacity) {
+        : capacity(newCapacity) {
       assert((size_t)capacity == newCapacity && "capacity overflow");
     }

It is likely an alignment issue lurking somewhere. The uninitialized region of the structure was partially overlapping, which aligns (ha ha) well here.