webadmin: Removed image resources no longer in use

The arrow images were removed from the VM boot sequence buttons, but
their ImageResource fetch methods were forgotten.

Change-Id: I34ee07c7cc0b9f10e08d2b59038bc7020f614f20
Bug-Url: https://bugzilla.redhat.com/1000285
Signed-off-by: Lior Vernia <lvernia@redhat.com>