[ADT] Remove ArrayRef(std::nullopt_t) (llvm#165831)

ArrayRef(std::nullopt_t) has been deprecated since:

  commit 2529de5
  Author: Kazu Hirata <[email protected]>
  Date:   Fri Jun 27 01:03:02 2025 -0700

Note that we've made at lease one release, llvmorg-21.1, with this
deprecation.

Author: kazutakahirata

llvm/include/llvm/ADT/ArrayRef.h

@@ -66,10 +66,6 @@ namespace llvm {
     /// Construct an empty ArrayRef.
     /*implicit*/ ArrayRef() = default;
 
-    /// Construct an empty ArrayRef from std::nullopt.
-    /*implicit*/ LLVM_DEPRECATED("Use {} or ArrayRef<T>() instead", "{}")
-    ArrayRef(std::nullopt_t) {}
-
     /// Construct an ArrayRef from a single element.
     /*implicit*/ ArrayRef(const T &OneElt LLVM_LIFETIME_BOUND)
         : Data(&OneElt), Length(1) {}