Stop configuring unused Ld command in `settings`

GHC has no direct dependence on the linker. Rather, we depend upon the C
compiler for linking and an object-merging program (which is typically
`ld`) for production of GHCi objects and merging of C stubs into final
object files.

Despite this, for historical reasons we still recorded information about
the linker into `settings`. Remove these entries from `settings`,
`hadrian/cfg/system.config`, as well as the `configure` logic
responsible for this information.

Closes #23566.