[GHC] #10913: deprecate and then remove -fwarn-hi-shadowing

It seems like we lost the code that -fwarn-hi-shadow switched during the add of hierarchical modules in 5.0. And while the hierarchical system makes the common way .hi files could get shadowed impossible, you can still get .hi file shadowing if you use the -i flag on multiple folders. Do we care? It is a potential source of very hard to find bugs, but it is also kind of a pathological case with how the modern module system works.