[flang] Fix Windows build (take 2)

Remove the offending "operator=()" member functions.  This should
silence the MSVC warnings that persist in uint128.h.

GitOrigin-RevId: ecc7f8ab74cdc106f2966597527f20a76118e27f
diff --git a/include/flang/Common/uint128.h b/include/flang/Common/uint128.h
index adbeeb3..b12e172 100644
--- a/include/flang/Common/uint128.h
+++ b/include/flang/Common/uint128.h
@@ -51,16 +51,6 @@
       : low_{n.low()}, high_{n.high()} {}
   explicit constexpr Int128(Int128<!IS_SIGNED> &&n)
       : low_{n.low()}, high_{n.high()} {}
-  constexpr Int128 &operator=(const Int128<!IS_SIGNED> &n) {
-    low_ = n.low();
-    high_ = n.high();
-    return *this;
-  }
-  constexpr Int128 &operator=(Int128<!IS_SIGNED> &&n) {
-    low_ = n.low();
-    high_ = n.high();
-    return *this;
-  }
 
   constexpr Int128 operator+() const { return *this; }
   constexpr Int128 operator~() const { return {~high_, ~low_}; }