Remove gtk_tree_view_set_fixed_height_mode() call from AssociateModel()

The check for wxDV_VARIABLE_LINE_HEIGHT is already done in Create(), and the
IsVirtualListModel() check was probably an attempt to get around a performance
problem (#16680), but it's not a valid criteria for changing the height mode.
closes #16683


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78170 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775