Revert "[ADT] Deprecate the redirection from SmallSet to SmallPtrSet (Take 2) (llvm#155078)"

This reverts commit 9b493dc.

There are hundreds of warnings when building LLVM/Clang because of this right now.
See the original PR for the detailed issues.

Also revert the follow-up fix "[ADT] Fix redirection of SmallSet to SmallPtrSet (llvm#155117)"
This reverts commit 3ca1ca4.

Author: joker-eph

llvm/include/llvm/ADT/SmallSet.h

@@ -268,17 +268,8 @@ class SmallSet {
 
 /// If this set is of pointer values, transparently switch over to using
 /// SmallPtrSet for performance.
-///
-/// We use this middleman class DeprecatedSmallSet so that the deprecation
-/// warning works.  Placing LLVM_DEPRECATED just before SmallSet below won't
-/// work.
-template <typename PointerType, unsigned N>
-class LLVM_DEPRECATED("Use SmallPtrSet instead", "SmallPtrSet")
-    DeprecatedSmallSet : public SmallPtrSet<PointerType, N> {};
-
 template <typename PointeeType, unsigned N>
-class SmallSet<PointeeType *, N> : public DeprecatedSmallSet<PointeeType *, N> {
-};
+class SmallSet<PointeeType *, N> : public SmallPtrSet<PointeeType *, N> {};
 
 /// Equality comparison for SmallSet.
 ///