remove seemingly bogus sigINT handler stuff

I am very doubtful that commit 613e747d91
was right about this doing anything, and I've verified that without it,
ctrl-c sends sigINT to child processes, and git-annex get does not
continue to the next item.

It seems likely that the real problem back then was something catching
the async exception.

Hard to see how installing a default signal handler could cause any
change from default behavior either.

One reason to want to get rid of this cruft now is that tasty has a
sigINT handler of its own, and this would override it.
(Tasty is not currently setting that handler up the way git-annex uses
it, due to a problem in tasty, but that will hopefully change.)

Annex/Action.hs

@@ -11,7 +11,6 @@ module Annex.Action where
 
 import qualified Data.Map as M
 #ifndef mingw32_HOST_OS
-import System.Posix.Signals
 import System.Posix.Process (getAnyProcessStatus)
 import Utility.Exception
 #endif
@@ -26,12 +25,7 @@ import Annex.CheckIgnore
 
 {- Actions to perform each time ran. -}
 startup :: Annex ()
-startup =
-#ifndef mingw32_HOST_OS
-	liftIO $ void $ installHandler sigINT Default Nothing
-#else
-	return ()
-#endif
+startup = return ()
 
 {- Cleanup actions. -}
 shutdown :: Bool -> Annex ()