Removed dead code in configure scripts.

Summary

I suspect we currently have a decent amount of dead code in configure scripts due to changes in the way we build GHC in recent years, especially the switch to Hadrian.

Steps to reproduce

git grep AC_SUBST | sed -E 's/^.*AC_SUBST\(\[?(.*)\]?\).*$/\1/' | sort | uniq

crudely gets a list of variables. Then, see if there are used in @...@.

For example, git grep '@CONF_LD_LINKER_OPTS' turns up nothing.

Expected behavior

What do you expect the reproducer described above to do?

Environment

• GHC version used:

Optional:

• Operating System:
• System Architecture: