Simplify av1_get_rest_tile_limits

The subtile and clamping features are no longer used. This patch
removes the dead code that implemented them and the parameters to
support them.

It also changes the return format. Instead of having return type void
and passing data out through 4 output pointers, the function now just
returns a RestorationTileLimits structure. Since the function is
defined inline in a header, I suspect that most callsites will
actually compile to identical code.

There should be no functional change from this patch.

Change-Id: I6ebc4da66a00676bd988f939a4b4957f743e8004