libutil: remove unused blockInt

this has been unused since ab3ce1cc13,
which is now almost ten years ago. surely it was just forgotten then

Change-Id: Ib93b52d4ce5f19629432c5f91ff426582c57f99b

lix/libmain/shared.hh

@@ -10,9 +10,6 @@
 #include "lix/libutil/processes.hh"
 #include "lix/libutil/strings.hh"
 
-#include <signal.h>
-
-
 namespace nix {
 
 int handleExceptions(const std::string & programName, std::function<int()> fun);
@@ -89,9 +86,6 @@ private:
     int std_out;
 };
 
-extern volatile ::sig_atomic_t blockInt;
-
-
 /* GC helpers. */
 
 std::string showBytes(uint64_t bytes);