Remove system-filepath dependency (dhall-lang#2502)

Fixes dhall-lang#2495

This dependency was completely unused

Author: Gabriella439

dhall/dhall.cabal

@@ -444,7 +444,6 @@ Test-Suite tasty
         quickcheck-instances      >= 0.3.12   && < 0.4 ,
         special-values                           < 0.2 ,
         spoon                                    < 0.4 ,
-        system-filepath                                ,
         tasty                     >= 0.11.2   && < 1.5 ,
         tasty-expected-failure                   < 0.13,
         tasty-hunit               >= 0.10     && < 0.11,