Drop `ld` information from `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.