ubifs: switch_gc_head: Remove redondant sync of wbuf

The wbuf is already sync-ed before ubifs_leb_unmap()

Signed-off-by: Sylvain Etienne <seti@dadboo.eu>
Acked-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>