glsl: slim the kill_entry struct used in const propagation

glsl: slim the kill_entry struct used in const propagation

Since 4654439fdd7 "glsl: Use hash tables for
opt_constant_propagation() kill sets." uses a hash_table for storing
kill_entries, so the structs can be simplified.

Remove the exec_node from kill_entry since it is not used in an
exec_list anymore.

Remove the 'var' from kill_entry since it is now redundant with the
key of the hash table.

Suggested by Eric Anholt.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>