[IR] Remove deprecated PointerType::getElementType()

This was deprecated before the LLVM 14 branch cut, remove the
method now.

As a temporary workaround, Type::getPointerElementType() can be
used instead.

See https://llvm.org/docs/OpaquePointers.html for information on
the opaque pointers migration.

llvm/include/llvm/IR/DerivedTypes.h

@@ -670,13 +670,6 @@ class PointerType : public Type {
     return get(PT->PointeeTy, AddressSpace);
   }
 
-  [[deprecated("Pointer element types are deprecated. You can *temporarily* "
-               "use Type::getPointerElementType() instead")]]
-  Type *getElementType() const {
-    assert(!isOpaque() && "Attempting to get element type of opaque pointer");
-    return PointeeTy;
-  }
-
   bool isOpaque() const { return !PointeeTy; }
 
   /// Return true if the specified type is valid as a element type.