video: adf: use rb_erase in adf_obj_destroy.

Not calling rb_erase() can cause slab corruption, as the rb_first() call
after kfree() in adf_obj_destroy() can return the same node twice unless
it is erased.

This problem was reproduced by unloading a kernel module that used the
adf framework *after* a vsync event was registered. A crash would occur
in rb_first(). (Just loading and immediately unloading the module without
the vsync event worked correctly.)

Change-Id: I9fa7cb5d7519691e38a281439844aa193da13d1b
Signed-off-by: Alistair Strachan <alistair.strachan@imgtec.com>
Cc: Jonathan Hamilton <jonathan.hamilton@imgtec.com>
Cc: Greg Hackmann <ghackmann@google.com>