Use wxFont::Bold() in wxGenericTreeCtrl instead of wrongly duplicating it.

Use wxFont::Bold() in wxGenericTreeCtrl instead of wrongly duplicating it.

Creating a new wxFont from the components of the existing one doesn't always
work because we may not have a valid value for the old font family so attempt
to use it as a family of the new font results in an assert.

Just use wxFont::Bold() instead to avoid the problem. It's much shorter and
obviously correct and doesn't result in asserts.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64649 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775