[PATCH] Remove obsolete tupDesc assignment in extended statistics

Hi hackers,

Attached is a small cleanup patch for extended statistics.

lookup_var_attr_stats() assigns tupDesc to VacAttrStats entries created 
for expressions. That appears to be obsolete: make_build_data() uses 
tupDesc only when fetching values for regular columns, while expressions 
are handled separately using VacAttrStats entries created by 
examine_expression().

The old comment also points at statext_mcv_build(), but this does not 
seem to match the current code anymore.  If some future code needs a 
tuple descriptor for expression entries here, it should probably set up 
that dependency explicitly rather than rely on copying vacatts[0]->tupDesc.

The patch removes the assignment and the outdated XXX comment.

Best regards,
Denis Rodionov
Tantor Labs LLC,
https://tantorlabs.com/