Silence unused variable compiler warning

Silence unused variable compiler warning

Fallout from fcec6caafa2: mark a variable in
set_tablefunc_size_estimates as used for asserts only.

Also, the planner_rte_fetch() call is pointless with assertions
disabled, so enclose it in a USE_ASSERT_CHECKING #ifdef; fix the same
problem in set_subquery_size_estimates().

First problem noted by David Rowley, whose compiler is noisier than mine
in this regard.