Remove wxBookCtrlBase::AcceptsFocus() returning false as it didn't make
any sense: even though wxBookCtrlBase doesn't, indeed, accept focus on
itself, it does accept it for its children and returning false prevented
the focus from ever getting inside it.
This fixes, for example, keyboard navigation in a window containing
wxSimplebook and TAB can now be used to move to the controls inside it
from the outside.
Also remove the now unnecessary AcceptsFocus() override which was just
undoing the damage of the base class method and is not needed any more.
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