obsolete reference to a SubPlan field

I noticed $subject while looking at something involving SubLinks and
SubPlans.  It seems eab6b8b27eb removed the "plan" field from the
SubPlan node struct definition, but the following line from
expression_tree_mutator():

                /* but not the sub-Plan itself, which is referenced as-is */

and the following from expression_tree_walker():

               /* recurse into the testexpr, but not into the Plan */

both of which I think refer to that no-longer-existent field, appear
to have survived multiple commits that moved the SubPlan expression
processing code around.

Attached patch removes those.

-- 
Amit Langote
EDB: http://www.enterprisedb.com