exec_node: Remove destructor from exec_node and all descendants.

exec_node: Remove destructor from exec_node and all descendants.

Two of these destructors are non-empty, (s_symbol and s_list), so this
commit could potentially introduce memory leaks, (though, no additional
leaks are found in glsl-orangebook-ch06-bump.frag at least---perhaps
the current code is never calling delete on these classes?).

Going forward, we will switch to talloc for exec_node so we won't need
explicit destrcutors to free up any memory used.