Fix wrong iterator in wxInfoBar::RemoveButton() in wxGTK

The iterator passed to erase() was off by 1, but this worked correctly
in the default build using wxVector, and not std::vector, because its
base() implementation was off by 1 too.

Now that wxVector bug is fixed (see the parent) commit, fix the code
using it too, which makes it work both in the default and STL builds.

Closes #18765.

src/gtk/infobar.cpp

@@ -323,7 +323,7 @@ void wxInfoBar::RemoveButton(wxWindowID btnid)
         if (i->id == btnid)
         {
             gtk_widget_destroy(i->button);
-            buttons.erase(i.base());
+            buttons.erase(i.base() - 1);
 
             // see comment in GTKAddButton()
             InvalidateBestSize();