[gui] Remove unnecessary AddReference()

[bellenot]

AddReference() is already called in TGPicturePool::GetPicture()

[linev]

Fix can be dangerous if we have something like:

auto pic1 = fClient->GetPicture("checked_dis_t.xpm");
auto pic2 = fClient->GetPicture("unchecked_dis_t.xpm");

item1->SetCheckBoxPictures(pic1, pic2);
item2->SetCheckBoxPictures(pic1, pic2);
item3->SetCheckBoxPictures(pic1, pic2);

I guess, original implementation was done for such usecase.
There is no such code in ROOT itself, but probably in users code.

[linev]

It is potentially dangerous change

[bellenot]

@linev you're right, and there are no such use cases in ROOT, but since there is no way to know how the pictures passed as argument were created (without changing the API), I'll let the code as it is now...

[linev]

Another solution - change caller code in ROOT.

auto pic1 = fClient->GetPicture("checked_dis_t.xpm");
auto pic2 = fClient->GetPicture("unchecked_dis_t.xpm");
parent->SetCheckBoxPictures(pic1, pic2);
fClient->FreePicture(pic1);
fClient->FreePicture(pic2);

Not elegant, but correctly handle references

[bellenot]

@linev OK, will implement this. And I see there are already a couple of cases where it's already like this.

[bellenot]

@linev superseded by #22046