Revert "[lldb][ObjC][NFC] Fix c++20 gcc compile errors"

This reverts commit 5e9f0e37
because it creates a new warning from clang:
```
NSDictionary.cpp:1063:14: warning: ISO C++ requires the name after '::~' to be found in the same scope as the name before '::~' [-Wdtor-name]
    D32, D64>::~GenericNSDictionaryMSyntheticFrontEnd() {
    ~~~~~~~~~^~
             ::GenericNSDictionaryMSyntheticFrontEnd
```

If you remove the template arguments from before the `::`, you
then get:
```
NSDictionary.cpp:1062:27: error: use of class template 'lldb_private::formatters::GenericNSDictionaryMSyntheticFrontEnd' requires template arguments
lldb_private::formatters::GenericNSDictionaryMSyntheticFrontEnd::~GenericNSDictionaryMSyntheticFrontEnd() {
                          ^
```

And I'm not aware of a way to fix that.