Remove unused file

common/src/GHC/Debug/Convention.hs

-
-module GHC.Debug.Convention (socketDirectory) where
-
-import System.FilePath
-import System.Directory
-
--- | The default socket directory in which to place unix domain sockets in
--- ghc-debug-stub. This is currently your XDG data directory.
-socketDirectory :: IO FilePath
-socketDirectory = do
-    xdgDir <- getXdgDirectory XdgData ""
-    return (xdgDir </> "ghc-debug/debuggee/")