Remove duplicate #define

Mark Dilger
Repair corner-case bug in array version of percentile_cont().

The code for advancing through the input rows overlooked the case that we
might already be past the first row of the row pair now being considered,
in case the previous percentile also fell between the same two input rows.

Report and patch by Andrew Gierth; logic rewritten a bit for clarity by me.
WIP
Change to rowshare
WIP