Drop SINGULAR_EXECUTABLE from src/bin/sage-env #32302
Description
It's unused. Doing this on a separate ticket from
#29024 and #32242 as it should be a trivial review.
CC: @mkoeppe @dimpase @kiwifb @antonio-rojas @slel
Component: build
Author: Michael Orlitzky
Branch/Commit: 2acc06e
Reviewer: Matthias Koeppe
Issue created by migration from https://trac.sagemath.org/ticket/32302