It turns out that _SmallBuffer<T> simply isn't used at all

... so that's another vestigial static-buffer we can eliminate.

Author: stephentyrone

stdlib/public/core/CMakeLists.txt

@@ -127,7 +127,6 @@ set(SWIFTLIB_ESSENTIAL
   ShadowProtocols.swift
   Shims.swift
   Slice.swift
-  SmallBuffer.swift
   SmallString.swift
   Sort.swift
   StaticString.swift

stdlib/public/core/GroupInfo.json

@@ -144,7 +144,6 @@
     "VarArgs.swift",
     "CTypes.swift",
     "MemoryLayout.swift",
-    "SmallBuffer.swift"
   ],
   "KeyPaths": [
     "KeyPath.swift"