trace-events: Drop unused events

trace-events: Drop unused events

Dropped event                           Unused since
mirror_cow                              884fea4
paio_complete                           47e6b25
paio_cancel                             47e6b25
usb_ehci_data                           0ce668b
megasas_qf_dequeue                      never used
megasas_handle_frame                    never used
megasas_io_continue                     never used
megasas_iovec_map_failed                never used
megasas_dcmd_map_failed                 never used
milkymist_softusb_mouse_event           4c15ba9
xen_map_block                           6506e4f
xen_unmap_block                         6506e4f
qemu_spice_start                        67be672
qemu_spice_stop                         67be672

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>