1. deprecate redundantly sounding wxArtProvider::FooProvider() to just Foo()

1. deprecate redundantly sounding wxArtProvider::FooProvider() to just Foo()
2. wxArtProvider::Remove() now really only removes the provider without
   deleting it, Delete() does delete it
3. moreover, ~wxArtProvider removes the provider as well so there is almost
   no need to call Delete() explicitly any more

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@41398 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775