nir/vars_to_ssa: Delete dead output set code

nir/vars_to_ssa: Delete dead output set code

This was a remnant of an early attempt to handle output reads in
vars_to_ssa.  That attempt was abandon a long time ago but these few lines
were aparently left in the pass and managed to evade review.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>