Review why `-fwarn-unused-binds"` flag is singled out in filtering

[hvr]

As discussed w/ @merijn I'm filing this ticket so this isn't forgotten

cabal/Cabal/Distribution/Simple/Program/GHC.hs

Line 170 in 50be57e

[ [ "reverse-errors", "warn-unused-binds", "break-on-error"

The -fwarn-unused-binds flag has been singled out ever since 20dcabf

[merijn]

Relevant IRC discussion:

<merijn> hvr: Basically, all the other warning flags were just a copied from a huge list of warning flags in the GHC commandline parser, but warn-unused-binds isn't in there
<merijn> It's listed separately, as one of it's variants is hiddenh
<merijn> https://github.com/ghc/ghc/blob/master/compiler/main/DynFlags.hs#L3636-L3639
<merijn> This somehow crept into my code, but there's no real reason
<merijn> Yeah, it should be safe to just ditch the entry for unused-binds, assuming those 4 GHC lines haven't really changed since GHC 8.0