Revert "Allow using bitfield's copy-assignment operator"

This reverts commit 9b7be93420.  Note that this reversion means code no longer compiles properly.  (Also note that BitField.h is still modified from the version used in Dolphin, to remove references to Inline.h.)

common/BitField.h

@@ -131,9 +131,13 @@ public:
   // so that we can use this within unions
   constexpr BitField() = default;
 
-  // Warning!  This copies ALL storage bits, not just the bits represented by this bitfield.
-  // This exists so that unions can be copyable.
-  BitField& operator=(const BitField&) = default;
+  // We explicitly delete the copy assignment operator here, because the
+  // default copy assignment would copy the full storage value, rather than
+  // just the bits relevant to this particular bit field.
+  // Ideally, we would just implement the copy assignment to copy only the
+  // relevant bits, but we're prevented from doing that because the savestate
+  // code expects that this class is trivially copyable.
+  BitField& operator=(const BitField&) = delete;
 
   BitField& operator=(T val)
   {